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By C. Rhodes

Liquid metal electro-chemical cells are of fundamental importance for stationary multi-MWh electrical energy storage systems. Liquid metal electro-chemical technology provides a practical means for utility scale daily storage and recovery of electrical energy. An important near term application for such batteries is for energy storage at rural electric vehicle charging stations which either have a limited feed capacity from the electricity grid or which rely upon wind and solar electricity generation for primary energy supply. A second potential application of this technology is for daily storage of electricity at times when it is cheap for later use at times when it is expensive.

Maintenance of good performance from a liquid metal energy storage system requires an electronic charge control system that periodically automatically disconnects individual cell strings from the battery and completely discharges and then recharges each such cell string to recuperate its constituant chemicals. When operated in this manner thermally stratified liquid metal batteries have almost no loss of capacity during thousands of deep charge-discharge cycles. The round trip energy storage efficiency is typically about 75%.

Presently the practical electro-chemical technologies for storing electrical energy for later recovery are lithium ion batteries, liquid metal batteries and vanadium flow batteries. Lithium ion batteries are believed to be too expensive and working life limited for large scale stationary energy storage. Vanadium flow batteries require significant pump maintenance and are both too bulky and too environmentally dangerous for widespread use in residential / commercial applications. Hence, this web page focuses on liquid metal electro-chemical energy storage technology. There are multiple potential liquid metal cell anode-electrolyte-cathlode material combinations. This web page focuses on Na-S-NiCl because of its relatively high energy density, relatively low operating temperature and modest material cost. However, it is possible that other material combinations may ultimately be commercially more successful, particularly in displacement of nickel.

The modern sodium-sulfur-nickel chloride energy storage cell is the result of about 50 years of patient research and development in the areas of sodium-sulfur batteries, sodium-chloride (zebra) batteries and density stratified liquid metal batteries. However it is only very recently that these technologies have merged into a potentially commercially viable technology. The purpose of this web page is to briefly describe one implementation of this merged technology. A leading technical authority on the merged technology is Prof. Don Sadoway. Video lecture on liquid metal batteries

The initial cost of implementing a liquid metal energy storage system is a strong function of the price of its constituant materials. Frequently the dominant cost is the cost of nickel. The other materials are comparatively inexpensive. The nickel and other cell materials are not consumed and with reasonable care can be reused indefinitely.

A liquid metal battery consists of a top layer of a low density low melting point metal from the left hand side of the chemical periodic table such as sodium, a middle layer of a meduim density electrolyte (ion transport liquid) and a bottom layer of higher density materials from near the right hand side of the periodic table such as nickel chloride and nickel. The electrolyte must transport metal ions but not transport electrons. Suitable electrolytes include liquid salts and/or liquid sulphur containing Na2S5++.

On discharge the main source of energy in Na-S-NiCl cell is conversion of elemental Na into NaCl. A secondary source of energy is conversion of elemental Na into Na2S5. These energy sources are partially offset by the energy required to convert NiCl2 into NiS5 and then convert NiS5 into Ni.

Older versions of sodium sulfur cells relied on a thin alumina electrolyte layer. Newer cell versions avoid the requirement for this separator by use of spontaneous material density stratification. However, use of spontaneous density stratification requires that the energy storage cells be stationary and have rigid horizontal mounting. Each cell needs a stable high density ceramic wall to prevent the liquid metal anode electrically shorting to the cathlode material. Air must be completely excluded from the cell, which requires a reliable seal that will withstand severe temperature cycling. During operation the cell should be maintained at the temperature (> 300 deg. C) at which the density of Na2S5 is approximately equal to the density of liquid sulfur.

It is important to operate an energy storage system in a manner that prevents formation of electrically conductive whiskers at the bottom cathlode which can potentially reach through the electrolyte and cause an internal electrical short by providing an internal electron conduction path between the top anode and the bottom cathlode metal layers.

Energy storage systems based on this technology can be fabricated in modules which lend themselves to truck or rail transport. Battery, inverter, transformer and switchgear components should be easily interchangeable for service. Presently the energy density is about 1 MWh / 10 tonnes. One 18 wheel boom truck load (40 tonnes) can consist of 4 MWh of Na-S-NiCl2 energy storage. There is ongoing development aimed at increasing the average energy density.

Liquid metal energy storage cells operate at temperatures above 300 degrees C. Some metal combinations operate at as high as 475 degrees C. The thermal insulation required to efficiently thermally isolate liquid metal batteries is bulky. Larger liquid metal batteries are usually more thermally efficient than smaller ones due to their larger volume to surface area ratios. Generally liquid metal energy storage systems are sized in 10 ton modules to permit convenient truck, container and rail transport.

A liquid metal cell typically consists of an outer electrically insulating cylindrical mount or enclosure, a high density closed thick bottom metal cylinder made of nickel which forms the cathlode, an inner cylindrical ceramic wall, a medium density molten salt or like liquid ion transport electrolyte such as sulfur and a low density top metal such as sodium which forms the floating anode. At the cell design operating temperature the cathlode, electrolyte and anode materials spontaneously density separate and stratify with the anode on top and the cathlode on the bottom. For the Na-S-NiCl2 system the electrolyte is a mix of S8 and Na2S5. This cell should be operated at a temperature > 300 deg C which keeps the Na2S5 mobile in the liquid S8.

At the cell outer perimeter the stratified liquid anode layer is electrically isolated from the cathlode by the cylindrical inner ceramic wall.

Air is excluded. Small amounts of other substances and/or an excess of nickel may be used at the cathlode to improve the cell electrical conductivity.

If the object is to make an electro-chemical cell with the highest possible energy content per unit weight, the periodic table of elements indicates that the cell's active elements must come from the top left and top right corners of the periodic table. On the top left are hydrogen, lithium, sodium and magnesium. On the top right are fluorine and chlorine. However, use of some of these elements is impractical for various reasons.

Over the temperature range of interest hydrogen is a gas that is difficult to store in quantity, except in a large, heavy and expensive pressure tank.

Lithium is not a common element and it is in high demand for use in automotive propulsion batteries, which makes it too expensive for most stationary energy storage applications.

Fluorine forms strong chemical bonds that are difficult to break for energy storage following energy discharge.

The next best choices for energy storage are sodium (Na) and chlorine (Cl). However, chlorine is a dangerous gas. The practical way to use chlorine is to weakly chemically bind it to another element. This element must be chosen so that the resulting compound does not spontaneously decompose at the highest anticipated operating temperature. Hence the chlorine is stored as nickel chloride (NiCl2).

In theory a simple Na-NiCl2 molten salt cell could be made which when it discharges forms NaCl and Ni. However, to enable ion mobiity such a simple cell must be operated above the melting point of NaCl (801 C). That temperature is too high to be practical.

One solution is to use sulfur to form a Na-S-NiCl2 cell. In this cell there are two ongoing reverseable chemical reactions of the form:
2 Na+ + 5 S = Na2S5++
Na2S5++ + NiCl2 = 2 NaCl + S5 + Ni++

Thus during energy discharge sodium in the anode gives up an electron to the external circuit and forms Na2S5++ ions which moves through the liquid sulfur and combines with NiCl2 to form NaCl, S5 and Ni++ near the cathlode. These nickel ions accept electrons from the external circuit and plate out onto the cathlode. The sulfur which is an electrical insulator acts as a pseudo-electrolyte which allows transport of sodium ions by forming Na2S5++.

During energy charging sodium ions from the NaCl again form Na2S5++, move across the sulfur layer and become elemental sodium at the anode. The chlorine ions near the bottom cathlode react with the nickel at the cathlode to form NiCl2.

The sodium, sulfur and sodium polysulfide all melt at less than 275 degrees C allowing these electro-chemical reactions operate efficiently at about 300 degrees C. The cell operation is optimal at the temperature at which the Na2S5++ is mobile in the S8. However, historically the problem with 300 degree C operation was long term formation of metallic whiskers that would short out the cell after about 2000 charge/discharge cyces. This problem appears to have been solved by use of suitable support electronics which periodically recuperate each cell by complete cell discharge. A company active in this area is a MIT spinoff named AMBRI. Some parties have claimed further cell performance improvements by addition of other materials to improve cell electrical conductivity near the cathlode.

The sulfur electrolyte has the property that it conducts Na2S5++ ions but does not conduct either electrons or neutral Na atoms. During cell discharge Na+ ions formed in the liquid Na combine with the sulfur to become Na2S5++ ions that pass through the sulfur layer. These ions then react with the chlorine in the denser NiCl2 to form 2 NaCl, liberating Ni which plates out onto the cathlode while absorbing free electrons. After all the NiCl2 is exhausted the remaining sulfur acquires more Na and fully converts to Na2S5.

During cell charging first the Na2S5 converts back to S8 and Na2S5++ which can then transport Na+ ions from the NaCl. The Na+ ions from the NaCl again become Na2S5++, pass back through the S8, acquire electrons and accumulate as elemental liquid sodium at the anode. The Cl- released by the NaCl combines with the cathlode's nickel to form NiCl2, releasing electrons to the external circuit. After all the NaCl is exhausted the remaining Na2S5 continues giving up Na until the Na2S5 has fully converted back to sulfur. The sulfur so released in addition to forming S8 can form NiS or Ni3S2 which must be subsequently eliminated via complete cell discharge. The primary role of the sulfur/sodium polysulfide is to act as an electrolyte, which transports Na+ ions between the liquid sodium anode and the reservoirs of solid NiCl2 and solid NaCl near the cathlode. The sulfur/sodium polysulfide also provides some energy storage. When the cell is nearly fully charged there is a thick layer of NiCl2 and NaCl over the cathlode the cell's internal electrical resistivity rises. When the cell is nearly fully discharged there is a thick layer of Na2S5 on top of the cathlode which again increases the cell internal reisitance. Hence some parties shape the cathlode and/or add small amounts of other high density electrically conductive materials to mitigate this increase in internal cell resistance. A better strategy is to operate the cell in the middle 80% of its capacity range where this issue of formation of high resistivity deposits on the cathlode is not a problem.

The main problem with this Na-S-NiCl-Ni cell is long term formation of high melting point NiS (797 deg C) and Ni3S2 (790 deg C) whiskers that do not disappear during normal charge-recharge cycling. To aviod this problem each cell must periodically be completely discharged to convert these whiskers back into NiCl2. Hence this cell type should be installed with support electronics that periodically selectively isolates and completely discharges each cell.

The Na-S-NiCl2 energy storage cell operates in two stages during both charging and discharging. There is a distinct change in cell voltage and cell internal resistance at the interstage transition. The interstage transition, in combination with a current integrator, can be used to estimate the remaining useful cell charge. In general the cell is most efficient when it is converting NiCl2 to 2 NaCl during cell discharge and when it is converting 2 NaCl to NiCl2 during cell charging. At the top and bottom ends of the charge-discharge range the internal resistance is higher and nickel sulfides form or are eliminated. The nickel sulfide formation/elimination process reduces the cell's efficiency. In applications involving high charge-discharge rates the cell should be operated in the middle 80% of its operating range where the internal resistance is relatively low and where formation/elimination of nickel sulfides is not an issue.

Initially, when the cell is fully charged, all the Na is in the anode and all the chlorine is contained in NiCl2. During the cell discharge each participating Na atom in the anode gives up an electron to the external circuit and forms an Na+ ion according to the equation:
Na = Na+ + e-
The Na+ ions react with the liquid sulfur to form Na2S++. The Na2S++ is unstable and spontaneously takes on more sulfur to form Na2S4++ or Na2S5++. These ions are known as sodium polysulfides. The net equation is:
2Na++ + 5S = Na2S5++
The Na2S5++ moves through the sulfur, finds a NiCl2 molecule underneath the sulfur layer and forms 2 NaCl according to the equation:
Na2S5++ + NiCl2 = 2 NaCl + 5 S + Ni++.
Each released Ni++ ion drifts to the cathlode where it neutralizes itself by acquiring two electrons from the external circuit and plates Ni metal onto the cathlode. This process continues until all the available NiCl2 is exhausted. The cathlode must be designed so that the Ni plating onto its surface does not significantly reduce its surface area.

During the discharge cycle second stage there is no NiCl2. The cell behaves as a pure Na-S cell. Na ions from the anode react with the sulfur. The sulfur, which during the discharge cycle first stage only partially converted to Na2S5 now continues to react with Na+ causing all the remaining sulfur to convert to Na2S5 according to the equation:
2Na+ + 5S = Na2S5++
Each Na2S5++ ion drifts to the cathlode where it acquires two electrons from the external circuit. This discharge process continues until all of the available sulfur has converted to Na2S5 which accumulates on top of the cathlode, increasing the cell internal resistance.

At that point there is no energy source to drive the external circuit and the cell is fully discharged. The cell internal resistance during the discharge cycle second stage is higher than during the discharge cycle first stage. There is a surplus of Na to ensure that the process is not Na supply constrained.

Initially, when the cell is fully discharged, all of the sulfur is contained in Na2S5, NiS, Ni3S2 and all of the chlorine is contained in NaCl. During the charge cycle first stage the Na2S5 at the Na2S5 top surface breaks down according to:
Na2S5 = 2 Na+ + 5S + 2e-
and the Na+ ions move back to the Na anode where they acquire neutralizing electrons from the external circuit. The two negative charges in the sulfur attract another two Na2S5++ ions from the NaCl reservoir, which restore the Na2S5 concentration, leaving Cl- ions. These Cl- ions drift to the Ni cathlode where they give up their electrons to the external circuit and combine with the nickel cathlode material to form NiCl2. This process continues until all of the available NaCl is exhausted. There is a surplus of Ni at the cathlode to ensure that the process is not constrained by the available supply of Ni.

The free Cl- ions may also combine with any NiS or Ni3S2 according to:
2Cl- + NiS = NiCl2 + S--
6Cl- + Ni3S2 = 3NiCl2 + 2S---
thus converting any NiS or Ni3S2 to NiCl2 plus free sulfur ions.
Since the sulfur is liquid the sulfur ions can migrate to the cathlode. There the sulfur ions do not chemically combine with the nickel during the charge cycle first stage because of the preference of nickel to combine with chlorine.

During the charge cycle second stage there is no NaCl. The cell behaves as a pure Na-S cell. The Na2S5 continues giving up Na+ ions which pass through the electrolyte to the Na anode. The negatively charged sulfur ions left behind circulate and give up their negative charge at the Ni coated cathlode and form the nickel sulfides NiS or Ni3S2. The negative charge flows in the external circuit. This process continues until all the Na2S5 is exhausted, at which point the cell is fully charged. The cell internal resistance during the charge cycle second stage is higher than during the charge cycle first stage due in part to accumulation of NiCl2 on the cathlode surface.

However, there is an issue with nickel sulfide formation during the charge cycle second stage. The sulfur and sodium polysulfide are liquids at 300 degrees C but the NiS and Ni3S2 are not liquids. Formation of nickel sulfides reduces the amounts of liquid sulfur and liquid sodium polysulfide that are available to provide cathlode structure conductivity. The nickel sulfides can be removed by completely discharging and then recharging the cell. However, while that process is taking place the cell is not available for random energy storage. Hence, a practical energy storage system involves multiple series connected strings of Na-S-NiCl2 cells. Then, with the aid of a suitable control system, each cell string can be completely discharged, partially recharged to clear any nickel sulfides, operated in an alternating charge-discharge mode and then charged only to the point where nickel sulfides start to form. A practical Na-S-NiCl2 energy storage system should have a microprocessor based charge controller to monitor and control the charging and discharging of each series connected string of Na-S-NiCl2 cells. This controller is essential for minimizing the nickel sulfide concentration in the cells and hence maximizing cell string performance.

The sodium in the anode must be above the Na melting point of 97.81 C in order to emit Na+ ions. The sulfur must be above its melting point of 112.8 C in order to allow Na2S5++ ions to circulate. The sodium polysulfide must be above its melting point of 275 C in order to circulate and form Na2S5++ ions. Hence in order to ensure proper operation the cell must be kept above 300 degrees C. The relative densities of liquid S8 and Na2S5 at the cell operating temperature are important. During cell discharge Na2S5++ ions must move downwards through the S8. During cell charging Na2S5++ ions must move upwards through the S8. These complexities tend to make liquid metal energy storage technology too complex for use by small consumers.

SodiumNa97.81 C.97
Sodium ChlorideNaCl801 C2.165-98.168 kcal/mole
Nickel ChlorideNiCl21001 C3.55-73.077 kcal/mole
Nickel mono-SulfideNiS797 C5.5
Nickel sub-SulfideNi3S2790 C5.82
SulfurS8112.8 C2.07
Sodium IodideNaI660 C3.67
Sodium HydroxideNaOH318.4 C2.13
Sodium MonosulfideNa2S1180 C1.856
Sodium TetrasulfideNa2S4275 C1.268
Sodium PentasulfideNa2S5251.8 C
NickelNi1455 C8.90
Nickel IodideNiI2797 C5.834


1) Due to their different densities the cell components tend to stratify. Over time the Ni cathlode tends to reform at the bottom of the cell. Stratification of components by density R in gm / cm^3 at 350 degrees C causes:
Na top R = 0.97
S8 next R = 1.66
Na2Sx next R = 1.85 to 1.91
NaCl next R = 2.16
NiCl2 next R = 3.55
NiS next R = 5.5
Ni3S2 next R= 5.82
Ni bottom R = 8.9

2) Note that the NaCl and the NiCl2 tend to sink in the liquid sulfur/sodium polysulfide. An important part of cell design is ensuring adequate fluid circulation adjacent to the cathlode structure. Note that Na2S5 and S8 are immiscible. If the temperature is not correct for Na2S5 transport in S8 additives are required to make sulfur electrically conduct. Note that at low temperatures the density of S8 is 2.07 and is denser than the Na2Sx. The temperature at which the density of liquid S8 equals the density of liquid Na2S5 likely represents the optimum cell operating temperature.

3) The net chemical reaction is:
2Na + NiCl2 = 2NaCl + Ni
Energy is mainly stored as sodium metal and as nickel chloride. Nickel chloride is used because it stores chlorine at the working temperature with minimum binding energy. Sulfur provides some additional energy storage at the expense of nickel sulfide formation which increases cell internal resistance.

4) The energy liberated per mole of NaCl is:
98.168 kcal/mole (73.077 / 2) kcal/mole = 61.63 kcal/mole

5) 61.63 kcal/ mole NaCl X 1 mole NaCl / (22.99 + 35.45) gm
=61.63 / 58.44 kcal/gm
=1.0545 kcal/gm X 1000 gm / kg = 1054.5 kcal/ kg
= 1054.5 kcal/ kg X 4.18 kJ/kcal
= 4407.8 kJ/kg X 1kW s/kJ X 1h/3600s
= 1.22 kWh / kg NaCl

6) Active chemical weight is further increased by nickel weight.
Total active chemical weight / NaCl weight
= (2(58.44) + 58.71) /2(58.44)
= 175.59 / 116.88
= 1.5023

7) Hence theoretical maximum energy / kg is:
1.22 kWh / 1.5023 kg
= .812 kWh / kg active chemical
By comparison the presently achieved total energy storage system weight is about 0.1 kWh / kg. Thus there is an opportunity for about a four fold increase in energy storage per unit weight via improved energy storage system engineering.

8) The theoretical minimum weight of nickel required to fabricate the energy storage cell is given by:
(1.22 kWh / kg NaCl) X ((2 X 58.44 kg NaCl)/(58.71 kg Ni)) X (.454 kg Ni) / (lb Ni)
= 1.102 kWh / lb Ni

Experimental experience indicates that the actual amount of Ni used is 3 times the theoretical minimum, indicating that the Ni requirement is:
3 lb Ni / 1.102 kWh = 2.72 lb Ni/ kWh
In early September 2009 the cost of Ni was about $8.12 US / lb, giving a cost of Ni for energy storage of:
$8.12 US / lb Ni X 2.72 lb Ni / kWh = $22.09 US / kWh
At $8.12 US / lb the cost of Ni is tolerable.

9) In industrial quantities the cost of sodium is about $4.00 / kg
In industrial quantities the cost of sulfur is about $.10 / kg

10) The cost of the chemical inventory required to store 1 kWh is:
= $25.00 / kWh of electrical energy storage capacity, which suggests that the large scale fabricated battery price will likely be in the range of $100 / kWh to $200 / kWh.

11) The basic energy storage cell components are:
a) ceramic cell enclosure
b) liquid Na anode
c) liquid S8/Na2S5
d) NaCl/NiCl2 reservoir
e) Ni cathlode
f) proprietary additives

12) The classic sodium=sulfur cell failure mode used to be formation of a metallic whisker that causes an internal short circuit. Use of superior charge control electronics appears to have solved this problem.

Practical energy storage systems contain multiple series connected strings of Na-S-NiCl2 cells. The minimum number of cells in a string is determined by the minimum required discharge voltage. Since the required charging voltage is higher than the available discharge voltage, generally two cell strings are series connected to obtain the required discharge voltage but are parallel connected for charging. For example an inverter that provides a 120V/208V 3 phase wye output can obtain its DC power from two series connected cell strings where each cell string has a minimum output voltage of:
1.41 X 120V = 169.2 volts.
Allowing for a 3.0 volt drop across switching devices and reactors increases this voltage requirement to 172.2 volts / cell string.

According to the published MES-DEA specifications the minimum operating voltage per cell is 1.722 volts, so the minimum number of cells per string is:
(172.2 volts/ cell string) / (1.722 volts / cell) = 100 cells / cell string

Application of similar design rules to a system with a 277V/480V 3 phase wye output results in a minimum output voltage per cell string of:
(1.41 X 277V) + 3V = 393.57 volts / cell string.
The corresponding minimum number of cells / cell string is:
(393.57 volts / cell string) / (1.722 volts / cell) = 228.5 cells / cell string.

Application of similar design rules to a system with a 347V/600V 3 phase wye output results in a minimum output voltage per cell string of:
(1.41 X 347V) + 3V = 492.27 volts / cell string.
The corresponding minimum number of cells / cell string is:
(492.27 volts / cell string) / (1.722 volts / cell) = 285.9 cells / cell string.

From the point of view of both electrical and thermal isolation it is convenient to package all of the cells of a cell string in a single module. From the point of view of practical handling with readily available lifting equipment, it is desirable to keep the maximum individual module weight less than 10 tons (20,000 pounds). Hence the individual cell weight including its enclosure allowance must be less than:
20,000 pounds / 300 cells = 66.67 pounds / cell = 30.2 kg / cell. An alternative is to design for 150 cells per module.

In order to meet charging/discharging voltage requirements a practical energy storage system has a minimum of 2 cell strings. In order to permit full charge-discharge cycling without imposing input or output constraints it is necessary to have additional cell strings. Generally there are either one or two modules per cell string. Each module string needs its own voltage and current monitoring for optimum charging and discharging.

Generally with Ni-S-NiCl2 energy storage systems, to accommodate the variable cell string voltages, there is one power inverter for each pair of cell strings. Each power inverter must be designed to accommodate the wide variations in DC bus voltages and to provide controlled rates of cell charging and discharging. Frequently there is a requirement for AC bus voltage control. The inverters must be fitted with integrators that monitor the Na-S-NiCl2 cell charge/discharge conditon to prevent over charging. The inverters must provide input/output signals to allow efficient control and monitoring of the energy storage system. There must be a means to ensure that the total load is appropriately and efficiently shared by the various inverters. There must be a means of automatically isolating a defective cell string or inverter such that the failure of a single unit does not cause a failure of the entire energy storage system.

Usually the energy storage system charging rate and discharging rate are controlled by an external energy management system that is programmed to minimize the facility owner's total electricity cost and/or maximize his distributed generation revenue. The energy transfer rate into or out of the energy storage system may be adjusted by changing the tap setting of the transformer between the AC line and the energy storage system.

These energy storage modules are heavy and should be installed in a location with easy boom truck access. Such locations have load bearing constraints. The support foundation must remain level during the operating live of the equipment.

Typically about 30% of the overall electrical energy that is input into the energy storage system is not recovered as electricity but instead is converted into heat. During the charging cycle there are rectification losses, switching losses and internal resistance losses. While energy is in storage there are continuous static heat losses. During the discharging cycle there are switching lossses and internal resistance losses. Ideally the energy storage system should be located such that the waste heat is recoverable and is used for a useful function such as space or domestic hot water heating. However, the energy storage system must be located such that at all times all of its heat output can be safely dissipated and such that it does not impose a fire risk. The combination of access, heat dissipation and fire protection requirements frequently makes practical heat recovery a challenge.

The energy storage system contains a substantial mass of a highly reactive liquid sodium at a high temperature. It is essential to prevent this liquid sodium reacting with either water or air. If water penetrates a cell seal large amounts of hydrogen will be released, that can easily lead to an explosion in a confined space. For this reason Na-S-NiCl2 energy storage modules should be installed in a well ventilated dry above grade location where flooding cannot occur. Similarly there should be an adequate fire separation between these energy storage modules and any combustable structure.

When it comes to batteries, lithium-ion are the best we have as far as energy density and convenience.

For now.
The Washington University in St. Louis lab of Peng Bai, assistant professor in the Department of Energy, Environmental & Chemical Engineering in the McKelvey School of Engineering, has developed a stable sodium ion battery that is highly efficient, will be less expensive to make and is significantly smaller than a traditional lithium ion battery due to the elimination of a once-necessary feature.

"We've found that the minimal is maximum," Bai said. "No anode is the best anode."

The research was published May 3, 2021, in the journal Advanced Science.

A traditional lithium ion battery consists of a cathode and anode, both of which store lithium ions; a separator to keep the electrodes separated on either side; and an electrolyte -- the liquid through which the ions move. When lithium flows from the anode to the cathode, free electrons leave through the current collector to the device being powered while the lithium passes the separator to the cathode.

To charge, the process is reversed, and the lithium passes from the cathode, through the separator, to the anode.

The concept of replacing lithium with sodium and doing away with the anode isn't new.

"We used old chemistry," Bai said. "But the problem has been, with this well-known chemistry, no one ever showed this anode-free battery can have a reasonable lifetime. They always fail very quickly or have a very low capacity or require special processing of the current collector."

Anode-free batteries tend to be unstable, growing dendrites -- finger-like growths that can cause a battery to short or simply to degrade quickly. This conventionally has been attributed to the reactivity of the alkali metals involved, in this case, sodium.

In this newly designed battery, only a thin layer of copper foil was used on the anode side as the current collector, i.e., the battery has no active anode material. Instead of flowing to an anode where they sit until time to move back to the cathode, in the anode-free battery the ions are transformed into a metal. First, they plate themselves onto copper foil, then they dissolve away when it's time to return to the cathode.

"In our discovery, there are no dendrites, no finger-like structures," said Bingyuan Ma, the paper's first author and a doctoral student in Bai's lab. The deposit is smooth, with a metal luster. "This kind of growth mode has never been observed for this kind of alkali metal."

"Observing" is key. Bai has developed a unique, transparent capillary cell that offers a new way to look at batteries. Traditionally, when a battery fails, in order to determine what went wrong, a researcher can open it up and take a look. But that after-the-fact kind of observation has limited usefulness.

"All of the battery's instabilities accumulate during the working process," Bai said. "What really matters is instability during the dynamic process, and there's no method to characterize that." Watching Ma's anode-free capillary cell, "We could clearly see that if you don't have good quality control of your electrolyte, you'll see various instabilities," including the formation of dendrites, Bai said.

Essentially, it comes down to how much water is in the electrolyte.

Alkali metals react with water, so the research team brought the water content down. "We were hoping just to see a good performance," Bai said. Watching the battery in action, the researchers shortly saw shiny, smooth deposits of sodium. It's the smoothness of the material that eliminates morphological irregularities that can lead to the growth of dendrites.

"We went back to check the capillary cells and realized there was a longer drying process of the electrolyte," Bai said. Everyone talks about the water content in batteries, but, in previous research, the amount of water had often been relegated to simply a statistic that needed to be noted.

Bai and Ma realized that it was, in fact, the key.

"Water content must be lower than 10 parts-per-million," Bai said. With that realization, Ma was able to build not just a capillary cell, but a working battery that is similar in performance to a standard lithium-ion battery, but takes up much less space because of the lack of an anode.

"Check your cell phone. Your electric car. One quarter of the cost of such items comes from the battery," Bai said. Sodium batteries use a more common metal than lithium batteries; they have the same energy density as lithium batteries; and they are smaller and cheaper than lithium batteries, thanks to the elimination of the anode.

"We proved you can use the simplest setup to enable the best battery," Bai said.

Story Source:
Materials provided by Washington University in St. Louis. Original written by Brandie Jefferson. Note: Content may be edited for style and length.

Journal Reference:
Bingyuan Ma, Youngju Lee, Peng Bai. Dynamic Interfacial Stability Confirmed by Microscopic Optical Operando Experiments Enables High-Retention-Rate Anode-Free Na Metal Full Cells. Advanced Science, 2021; 2005006 DOI: 10.1002/advs.202005006 Cite This Page:
Washington University in St. Louis. "Stable, efficient, anode-free sodium battery: One-of-a-kind tool helped solve anode puzzle that thwarted previous attempts." ScienceDaily. ScienceDaily, 3 May 2021.

This web page last updated May 4, 2021.

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