Solid state batteries for green mobility – understanding challenges and recent developments

Introduction

All-solid-state rechargeable lithium batteries (SSBs) have attracted much attention globally because of following key reasons:

– SSBs replace organic liquid electrolyte in Lithium-ion battery (LIBs) with a safer and more reliable inorganic solid electrolyte (SE), which simplifies the battery design and improves safety and durability of the battery. 

– Provision for direct stacking of solid-state cells in SSBs achieves a high operating voltage and compact size, with direct applications and benefits in Electric Vehicles (EVs). 

– Possibility to use large-capacity electrode materials, which the conventional LIBs cannot offer.

– SSBs also facilitate usage of solid electrolytes with high Li+ ion conductivity at room temperature. Inorganic SEs have been widely studied in SSBs and in recent years, several SEs have been developed with similar levels of conductivity as organic liquid electrolytes. Several techniques have been developed in SSBs over the last few years for increasing contact area at the solid electrode-solid electrolyte interface (SESEI) to enhance utilization of active materials and rate capability.

Solid State Batteries

The evolution of volumetric and gravimetric energy densities of LIBs, as shown in Figure 1 (Ref [1]),  indicates peak values of up to 770 Wh/litre and 260 Wh/kg, respectively. There is an ever-increasing demand for batteries with even higher energy density as well as high power density to enable quick charging and discharging capability. Solid-state batteries (SSBs) that use solid electrolytes (SEs) instead of liquid ones could offer both high energy and high-power density. 

In Figure 2, the architectures for Li-ion battery (middle), Li-metal based SSB (left), and Li-ion based SSB (right) are presented. It is observed that as we move from LIB in the middle towards LiM-SSB on the left, the volumetric and gravimetric energy densities (w_vol and w_grav,) increase by 70% and 40%, respectively. Similarly, as we move from LIB in the middle towards Li-SSB on the right, w_vol and w_grav, change by 0% and -10%, respectively.

Increasing the cell voltage from 4.2 V to 5 V would result in an increase of more than 20% in energy density (depending on additional cathode capacity at high voltage), and thus the use of a SE is favourable if it allows the use of either high-voltage cathodes (for example, materials with a 5 V redox potential) or high-capacity materials such as lithium metal that cannot be used in combination with liquid electrolytes. Oxide- or phosphate-based solid electrolytes are expected to chemically withstand high voltages, while liquid electrolytes are not stable at these voltages.

Figure 2 Middle: The conventional Lithium-ion Battery (LIB), shown in the middle in Figure 2, contains a liquid electrolyte, a porous anode (negative electrode) typically made of graphite, represented by grey circles, and a porous cathode (positive electrode) typically made of a layered transition metal oxide, represented by violet circles. The anode and cathode act as ‘active’ storage components, coated on thin foils serving as current collectors, with copper foils used for anode and aluminium foils used for cathode. A thin separator, typically of 10 μm thickness (represented by grey band in the middle in Figure 2), is placed between the much thicker electrodes (each about 100 μm thick). The liquid electrolyte infiltrates the porous electrode and separator assembly, providing fast ion transfer between the electrodes and preventing electronic short-circuiting. 

Figure 2 Right: In LI-SSB with a conventional anode, the liquid electrolyte in the electrodes is completely replaced by solid electrolyte (dark orange circles) with 0% change in w_vol. The electrodes and the electrolyte-separator are made of the same solid electrolyte (orange circles), resulting in 10% drop in w_grav.

Figure 2 Left: In LiM-SSB shown on the left, up to 40% increase in w_grav is achieved using a lithium-metal anode (light yellow) that has a theoretical energy density of 3,700 mA/g. Changes in energy density are estimated based on the density increase from liquid to solid, taking into account the high specific capacity of lithium metal and complete replacement of the graphite and anode electrolyte, leading up to 70% increase in w_vol.

For successful design, manufacture, and launch of SSBs, many challenges remain in both technological maturity and manufacturing readiness, including: 

a) what kind of benefits in volumetric and gravimetric energy densities can be achieved using SSBs,

b) what type of Solid Electrolytes (SEs) can be used in SSBs, 

c) what are the different materials and compositions of solid electrodes that can be used in SSBs

d) what are the potential solutions to overcome the inherent mechanical stability issues of SSBs, and 

e) how do we ensure safety of SSBs for mobility and energy storage industries

Next, the above challenges are detailed below, and the recent developments are presented. 

Solid Electrolytes (SE)

Most of the commercially available batteries use organic liquid electrolytes, which pose safety hazards due to inherent combustible nature though these batteries are cost effective and easy to manufacture. The biggest advantage of solid electrolytes (SEs) over liquid counterparts is overcoming unwanted chemical reactions leading to performance degradation, stability, and safety concerns. In batteries with liquid electrolytes and lithium-sulphur (Li-S) or lithium-metal (Li-M) electrodes, there is a high possibility of diffusion of soluble electrode components.

The challenges in Li-S cells include a) high cell polarization and under-utilization of active materials as sulphur is electrically insulating, b) polysulphide shuttle effect, which is caused by the dissolution of intermediate lithium polysulphide species in the electrolyte leading to irreversible loss of sulphur, and resulting in rapid capacity fading and poor Coulombic efficiency of the cells, and c) huge volume expansion of sulphur during repeated charging/discharging processes, disrupting the structural integrity of the electrodes and leading to poor electrical contact with the conductive additives and current collectors. The challenge with liquid electrolyte in Li-Metal cells is transition metal leaching. Additionally, LIBs with liquid electrolytes experience severe concentration gradients of the conducting salts during current flow with cell current limitations due to high mobility of lithium ions and anions in liquid electrolytes. 

SEs, on the other hand, allow transfer of lithium ions only and act as functional separators with only minor self-discharge. Also, bulk polarization cannot occur in SEs as only lithium ions are mobile, leading to the possibilities of higher current densities and quicker charging times in SSBs. Further, solid electrolytes are typically stable at elevated temperatures as their conductivity increases with increasing temperature, thus improving battery safety. Additionally, the mechanical rigidity of SEs may prevent the dendrite formation that is caused by the electrodeposition of lithium, and thus facilitate the use of lithium-metal anodes with potential for higher energy densities (LiM-SSB, Figure 2, left).

Two major classifications of Solid Electrolytes (SEs) exist — organic solid polymers and inorganic solids in either crystalline, glass or glass-ceramic in nature. Organic solid polymer-based electrolytes experience elastic and plastic deformation and thus can compensate for volume changes of electrodes, making them the preferred choice for use in SSBs. However, their main drawback is significantly lower lithium-ion conductivity for battery operation at room temperature. Hence, most of the research is focused on identifying stable polymer electrolytes for use with lithium-metal anodes and lithium nickel cobalt manganese oxide or lithium nickel cobalt aluminium oxide cathodes at ambient temperature at sufficient C-rates. Though some of the recent research has successfully demonstrated improved battery performance at high currents and good conductivity at room temperature using inorganic SEs, the major drawback of many inorganic SEs is their low thermodynamic stability. Akin to liquid electrolytes, protecting interphases are required to stabilize the electrolyte/electrode contact in SSBs as most SEs experience reduction at low potentials and oxidation at intermediate potentials. The highest lithium-ion conductivities in the solid state are offered by inorganic crystalline SEs such as oxides and thiophosphates. While thiophosphates are ductile and easily form dense cathode composites, oxides are brittle and often experience mechanical failure through cracking.

Key Performance Indicators (KPIs) for SSBs

For SSBs to be serious contenders to LIBs, which are continuing to improve in performance with further reduction in cost/kWh, they need to show a significant performance jump in one or more of the key properties such as energy density, power density, long-term stability, safety, and cost. Though energy density is the top priority for SSBs to compete with LIBs, power density is also important for quick charging. Next, long-term stability is important in terms of both long cycle and calendar life as the volume changes of the electrodes during cycling of the SSB cause mechanical strain and stability problems, directly impacting the lifetime of batteries in terms of number of discharge/charge cycles and time after production (calendar life). As SSBs aim to further improve the energy and power density of batteries compared to LIBs, safety is an important performance indicator for SSBs. Finally, the cost per kWh of energy storage capacity, highly dependent on production technology, is also important for SSBs to compete with LIBs (Ref [2]). 

a) KPI – Energy Density

If only the liquid electrolyte of an LIB is replaced by a SE (Figure 2, right), there is 0% change in w_vol. As SEs have a higher density than liquid electrolytes, there is about 10% reduction in w_grav. By using cathode materials of high-capacity or materials that can withstand 5V redox potential and by increasing the cell voltage from 4.2 V to 5 V, up to 20% increase in energy density can be achieved in SSB cells. SEs containing oxides or phosphates are expected to chemically withstand high voltages as opposed to liquid electrolytes. SEs also enable usage of high-capacity lithium-metal anodes, as their mechanical strength may prevent dendrite growth (Figure 2, left). As most lithium SEs are thermodynamically unstable against lithium metal, protective films, or a stable solid electrolyte interphase (SEI) are required to overcome this. In SSBs, a much thinner lithium layer (approx. 20 μm thick) is needed to replace the much thicker standard graphite anode of a typical LIB thick-film cell. Even with this thin Lithium film, inhomogeneous dissolution and subsequent deposition of lithium metal causes severe mechanical strain in SSBs which may lead to mechanical failure. In crystalline solid electrolytes, lithium dendrites can grow along grain boundaries and lead to fast short-circuits, and the cyclic dissolution and deposition of the metal at SEs needs to be fast enough to allow acceptable C-rates for charging/discharging. As can be seen from Figure 2 (left), LiM-SSBs containing Li-M electrodes operate well with low area-specific capacity and up to 70% increase in w_grav, where only about 1 μm of lithium is cycled—offers promising solution for future SSBs.

b) KPI – Power Density

Power density is a measure of how quickly batteries can deliver stored energy, how well they can handle high currents during faster charging or discharging, what impedance values result in cells, and what levels of material degradation and heat release occur in electrodes. With higher thermal conductivities and better thermal dissipation in solids compared to liquids, hot spots can be prevented in SSBs. An important design criterion in SSBs with highly conductive SEs is the cell kinetics at the solid electrode — solid electrolyte interface (SESEI), which in turn depends on the cell manufacturing process and quality. The study of cell kinetics at SESEI in SSBs is still preliminary and evolving. As bulk polarization is not possible in SEs unlike in liquid electrolytes, SSBs allow higher current densities and higher operating temperatures. However, issues may still arise at solid electrolyte/cathode interface due to the formation of lithium depletion layers in the SE, blocking cell kinetics in one direction at high potentials (similar to a diode), thus requiring additional interface protection. Research is still ongoing on fast cell kinetics at anode-SE-cathode interfaces with favourable design expectations to leverage the key advantages of SSBs.

c) KPI – Cyclability & Long-Term Stability

Though the long-term stability of SSBs is one of the major incentives for their development, significant research is still being carried out to better understand their electrochemical window (measured using cyclic voltammetry, Ref [2])) and the relatively small thermodynamic range of stability. As SEs typically use sulphides and thiophosphates, they react with lithium metal upon contact and form an interphase. The long-term operation and cyclability of SSB is assured if a highly conductive solid electrolyte interphase (SEI) is formed. Typically, mixed/average conductivity of SEI leads to fast deterioration of SSB. Interfacial instabilities may also occur on the cathode leading to degradation in SSBs, similar to LIBs. Though coating of cathode provides long-term stability allowing high-voltage operation and facilitating high-energy cells, the stability of the interphases remains challenging. Unlike LIBs, SSBs with SEs prevent unwanted electrode ‘cross-talk’, but still the extraneous outputs from side reactions form at SEI, impacting cell kinetics and cyclability. More research is being carried out to better understand the kinetic instability of lithium anode itself due to the pore formation during discharge and the dendrite formation during charge, to improve long-term stability of SSBs.

d) KPI – Safety

The safety of SSBs would be assured if only oxide ceramic components, which are inherently stable against lithium, were used. However, as the best-suited ionic conductors are sulphides, the formation of toxic H₂S in case of battery damage or the formation of SO₂ by oxidation of the solid electrolyte may present a potential risk (Ref [2]). Recently, garnet-type crystal Li₇La₃Zr₂O₁₂ (LLZO) has been attracting much attention as a crystalline SE because of its high conductivity of 3 × 10−4 S/cm and high chemical stability against lithium negative electrode (Ref [2]). However, oxide SEs are not as mechanically soft and lead to short-circuits and mechanical failure as dendrites may enter into polycrystalline solids along grain boundaries. Sophisticated protection techniques may be required, which complicates production and increases costs. 

e) KPI – Architecture and Manufacturing of SSB

Over the past decade, costs of LIBs have been dropping significantly with recent cost per kWh around $145, with further drop in cost expected to reach $100/kWh in the next few years. No reliable cost estimate for SSB production is available yet. Approximately $30/kg material and manufacturing costs at cell level are projected for the cost of $100/kWh in LIBs (Ref [3]). For SSBs to match the cost of LIBs with same material cost, they will have little room because of complex materials processing in SSBs. 

Figure 3 shows a schematic architecture of a single cell in an SSB. Owing to its high specific capacity, the use of a lithium metal anode can significantly increase the cell energy density. However, major challenges exist in developing SSBs, such as a) a resistive SEI may form between the lithium anode and the SE, b) resistance formation between the grains and grain boundaries of the SE, c) risk of short-circuiting from dendrite formation due to highly inhomogeneous lithium metal deposition. Coating and protection of electrode active materials is required as most SEs react with cathode active materials. Coating also helps prevent potential Li depletion at the cathode (space charge with rectifying effect). The red curve in Figure 3 (Ref [1]) indicates schematically the drop of the electric potential φ across the space charge. While the use of liquid electrolytes ensures good electrode contact and efficient ionic pathways, a solid electrolyte will typically have a microstructure consisting of small grains that must be in good contact with the electrode material as well as other electrolyte grains. Additionally, in SSBs, solid composite electrodes must be formed to ensure sufficient electronic and ionic percolation and establish an independent network of electronically conducting particles. As intercalation electrodes are known to exhibit volume changes, the mechanical integrity of the solid composite is a critical issue. The blue arrows in Figure 3 (Ref [1]) indicate that mechanical pressure is required both to avoid contact loss due to local volume changes upon lithiation/de-lithiation and to achieve high power density. In a liquid electrolyte, the electrode can ‘breathe’ and cycle reversibly. The challenges in solid electrolyte/electrode include increased interfacial resistance, over-voltage, and capacity fading as pores can form and cracking and loss of particle contact may occur in solids. It is helpful to use mechanically soft ionic conductors such as sulphide and thiophosphate glass SEs in SSBs. 

The manufacture of conventional LIBs involves separate lines of manufacture for anodes and cathodes coated onto foil current collectors, which are then integrated with a polymer separator, followed by various packaging operations, including injection of the liquid electrolyte. The serial production of LIBs results in high productivity though it involves a large number of steps. So far, the manufacturing approach for SSBs has followed a similar approach of discrete manufacturing processes for anode, cathode, and electrolyte; however, the electrolyte tends to be formed first, and the positive electrode (a powder-based composite of the active cathode material, carbon, and the solid electrolyte) and the Li-metal negative electrode (anode) are then added in separate operations. As the SE requires mechanical mixing and as the particle contacts are extremely sensitive to mechanical effects, SSBs will require a revolutionary change of production technology. The electrolyte, usually either an oxide (e.g., Li₇La₃Zr₂O₁₂ (LLZO)) or a sulphide (e.g., Li₆PS₅Cl), is generally required to be largely pore-free to maximise ionic conductivity. Sulphides offer a manufacturing advantage because they can be pressed to a high density at room temperature. In contrast, oxides tend to require relatively high process temperatures (up to 1000 ◦ C or even higher) and pressures (up to 500 MPa) for up to several hours’ sintering for useful density and ionic conductivity. When the oxide electrolyte is mixed with carbon and an active material, and consolidated to form a positive electrode, these high pressing temperatures tend to lead to excessive reactions and burn out of the carbon. Both oxides and sulphides exhibit sensitivity to water vapour, with LLZO forming unhelpful but essentially benign Li₂CO₃ whereas sulphides generate highly problematic toxic H₂S gas. Thus, particularly for sulphide based SSBs, manufacturing must be performed in a dry room or under inert atmosphere.

Conclusion 

Significant progress has been made on SSBs over the last few years, building upon past few decades of research on LIBs. However, the understanding of interfacial reactions and cell architecture and performance are still at an early stage. SSB development may be prompted by the superior kinetics when using intercalation anodes.

Three main points can be pointed out from SSB development till date: 1) optimization of poor interfacial kinetics between solid electrolyte and active materials requires more investigation, 2) mechanical pressure is required to guarantee stable operation of a solid-state cell with additional modelling and quantitative studies, and 3) LiM-SSB achieves significant improvement in energy density using lithium-metal anode. 

Sulphide electrolytes have several advantages of high conductivity, single Li+ ion conduction, wide electrochemical window, and intimate solid/solid contact. An advantage in SSBs is the possible use of active materials with large capacity such as sulphur and lithium metal, which are not available in conventional LIBs using liquid electrolytes. As solid electrolytes allow high current densities without concentration polarization, high power densities may be achieved in SSBs with intercalation anode and cathode.

Future Work 

Future research should focus on further increase of Li+ ion conductivity and chemical stability of sulphide electrolytes, and the formation of electrode/electrolyte interface achieving rapid charge transfer. For success of SSBs, it is also important to have control of size, morphology, and dispersibility of both SE and active material properties for better cell kinetics at SEI. For further increase in energy density and power density of SSBs, researchers should focus on how to increase the amounts of active materials in the composite electrode layer. 

References

1. Janek, J., Zeier, W. “A solid future for battery development,” Nature Energy, Vol. 1, 16141 (2016). https://doi.org/10.1038/nenergy.2016.141

2. Tatsumisago, M, et. al, “Recent development of sulfide solid electrolytes and interfacial modifications for all-solid-state rechargeable lithium batteries,” Journal of Asian Ceramic Societies, Vol. 1, 2013, pp. 17-25. http://dx.doi.org/10.1016/j.jascer.2013.03.005

3. Troy, S. et. al, “Life cycle assessment and resource analysis of all-solid-state batteries,” Nature Energy, Vol. 1, 16141 (2016). https://doi.org/10.1038/nenergy.2016.141

About the author

Dr. Arunkumar M. Sampath works as a Principal Consultant in Tata Consultancy Services (TCS) in Chennai. His interests include Hybrid and Electric Vehicles, Connected and Autonomous Vehicles, Cybersecurity, Extreme Fast Charging, Functional Safety, Advanced Air Mobility (AAM), AI, ML, Data Analytics, and Data Monetization Strategies.