On January 3 2024, VW’s PowerCO SE group released its findings on QuantumScape’s solid-state battery cell confirming it can hit 1,000 charging cycles and still maintain 95% original capacity. While this isn’t new as QuantumScape (QS) themselves have publicly announced this result in their Q3 2023 shareholder letter, this confirmation by a partner gives confidence that the science of QS’s battery is sound and promising. The PowerCO press release stated: “Depending on the model, an electric car could drive more than 500,000 kilometers without any noticeable loss of range.” 500,000 km is equivalent to 310,000 miles.
Lithium Ion Battery Basics
There are three parts in a lithium ion battery. Higher energy section, separator and lower energy section make up the battery. The components of a lithium battery can be explained by using the hydro power dam analogy. Water in the upper reservoir sits at a higher elevation and is separated from the lower reservoir by a dam. When the dam is turned on to release water, gravity pulls water down from upper to lower reservoir. The water flow spins a turbine to generate electricity. In a lithium ion battery, the upper reservoir is the anode (-), the separator in the middle is the equivalent of the dam, and the cathode (+) is the lower reservoir. When fully charged, the lithium is stored in the anode. When a switch is turned on connecting anode’s current collector with a load or device and the cathode’s current collector, the lithium atoms each give up one electron to become lithium+ ions. The lithium+ ion moves across the separator from anode into the cathode. The electron flows in the wire from the anode to the load/device and perform electrical work and ends up in the cathode’s current collector. During charging, the lithium+ leaves the cathode through the separator and joins with an electron provided by the charger and forms lithium atom in the anode.
QuantumScape Solid State Battery
A single layer or unit stack of QuantumScape’s solid-state battery starting from the bottom up consists of the cathode current collector, cathode, solid-state separator, empty anode-free layer (during manufacturing), and anode current collector. Two layers make up one unit cell with a shared cathode current collector in the middle and the anode current collector at top and bottom. (Source QuantumScape August 2023 shareholder presentation slide 13). The cells tested by PowerCO are 24 layers. PowerCO’s announcement shows that they have independently confirmed the cell’s performance characteristics. What this means for consumers is:
1. longer range EVs
2. faster charge times
3. better fire safety since the solid-state separator is thermal stable at higher temperatures.
Solid State Battery Advantages
Faster charge speed
During lithium ion battery recharging, the lithium ion in the cathode travels up through the separator to the anode electrode. Like a car traveling, the shortest path is a straight line. Inside the cathode however are lots of randomly placed electrode material and the lithium ion must go around them, taking them longer time and generating heat to make the trip. The other limiting factor is the anode material and separator has to be able to handle the large flow of lithium ions. In energy dense cells, graphite is often used in the anode and it is a bottleneck in charging. The lithium ion intercalate into the graphite during charge. There is a rate at which lithium can enter or diffuse into the graphite structure or atomic lattice. The faster rate is when battery is heated to 35-45 C. When a Tesla car navigate to a Supercharger for fast charging, the car preheats the battery on its way to the charging station. More on that later. If charging speed exceed this rate, the lithium ion sits on top of the graphite and can’t get into the atomic lattice. This is bad for the battery. The lithium now is stuck on the graphite and no longer available for work and battery capacity is reduced. The lithium plating on graphite can form dendrites that can reach from anode to cathode and short circuit the battery. The lithium in contact with liquid electrolyte form products that cover the graphite and also restrict lithium entry and lower charge rate. Silicon can be a substitute for graphite but silicon expands and contracts during charge and discharge and limits cycle life.
The preheating of the battery for optimal charge rate also promotes the chemical reaction between electrolyte and the electrode. This undesirable reaction takes away usable lithium produce extra undesirable material inside the battery that increase resistance. With lithium lost in the reaction, the battery capacity is reduced and thus battery life is cut short. If an EV driver occasionally fast charge, it’s fine but if she exclusively fast charge then over time the range will be reduce noticeably.
QuantumScape’s solid state battery does not have graphite host anode material. The lithium path is easier. The lithium atom does not need to find a graphite host lattice and diffuse into it. There is no diffusion rate limit and this makes charging faster. Without graphite present, there is also no reaction with graphite that would take away lithium atoms. It has no electrolyte and thus no by products during charging. QuantumScape’s separator material has good stability with lithium. There’s limited reaction and by product formation. There is still some reaction that may be why it dropped to 95% after 1000 cycles. The battery therefore can achieve fast charging with little degradation over hundreds of cycles.
QuantumScape’s white paper on charging speed has a graph showing charging speed at 25 C and 45 C. Starting at 10% charged, at 25 C, its cell can charge at 4C for a few minutes and start to slow and reaches 80% charged in about 13 minutes. At 45 C, its cell can sustain 4C charge rate for close to 10 minutes and reach 80% in 12 minutes. A conventional lithium ion battery takes about 30 minutes to reach 80% state of charge (SOC).
Higher Energy Density – Longer Range EV
For a given volume, solid state battery will provide more energy. In other words, its energy density is higher than traditional lithium ion battery. It achieves this because the anode is pure lithium vs in a traditional lithium ion battery, it has carbon + lithium or carbon + silicon + lithium.
Safer
In traditional lithium ion battery, if there is excessive heat at 70 C, the Solid Electrolyte Interface (SEI) layer that separates the electrolyte from the anode starts to decompose. At 130 C, the separator melts and creates short circuit. The short circuit release more heat as high current flows. At 200 C, the electrolyte decomposes. At 300 C, the cathode decomposes and lead to thermal runaway. The cathode decomposition releases Oxygen which is a fuel for the fire. NMC111 (nickel manganese cobalt oxides) has an onset temperature around 220 C and is exothermic (heat generating). LFP (Lithium Iron Phosphate) battery chemistry has higher onset temperature 230 C and is much less exothermic than NCM. An EV’s battery management provides thermal management to keep battery in optimal environment. It is also recommended to keep state of charge between 20-80% for NCM chemistry. This helps protect the SEI layer that can be damaged from overcharging.
Heat is released at high temperature when lithium reacts with the liquid electrolyte used in traditional lithium ion battery. They react above 176.0 C and get to peak heat/energy around 180 C, near the melting point of lithium. QuantumScape’s solid state separator on the other hand does not release energy and does not appreciably react with molten lithium metal according to its company’s 2020 presentation. There is some reaction at round 179.9 C but it actually consumes small amount of heat and does not release any heat. QuantumScape states, the solid state separator does not combust and has high thermal stability and that the lithium anode is stable with the separator. This property makes QuantumScape’s battery safer.
Challenges
The challenge and next step are to improve the manufacturing process to make these cells at high volume efficiently. The other challenge is to work within the cash QuantumScape has available and timeline the cash provides or runway. Getting funding could be difficult in a tough economic environment and when investors are risk averse.
Silicon Anode is a competing solution to replace the anode. It too offers a possible path to improving energy density, charger time. Sila Nano has gone through thousands of iterations of silicon to arrive at solution that can be a drop in replacement for carbon. There are 6 carbon atoms to store one lithium atom in a traditional lithium ion anode. Silicon can store 4 lithium atoms. Hence the lithium density limit is much higher with Silicon. The challenge with Silicon is that it will expands up to 300% and this expansion limits it cycle life. Tradition lithium battery approaches a limit of $100/kWh. Sila Nano see a path where price drops to $50/kWh. Their engineered silicon anode which they named Titan Silicon anode has 20% more energy density and swells 6% according to its website: https://www.silanano.com/our-solutions/titan-silicon-anode
NASA is also interested in researching solid state batteries. They need batteries to power an astronaut’s suite, electric avionics, rovers and robotics. Their research resulted in a Selenium Sulfur cathode. Sulfur has low conductivity 1 x 10-18 S/cm but the SeS2 compound is reasonably conductive. Their cathode is Holey Graphene combined with SeS2. The mixture has holes in the graphene and allow fast transport of ions. NASA’s battery has a solid electrolyte made up of ceramic solid state electrolyte and Li salt. According to NASA, it has ionic conductivity of 1.2 x 10-3 S/cm and is non flammable.
Lyten has 3D Graphene product and they are creating batteries similar to NASA’s design call Lyten Lithium Sulfur battery. Their cathode does not have Nickel, Manganese, and Cobalt (NMC). They claim it will offer 2x the energy density of traditional lithium batteries. Lyten was founded in 2015 and headquartered in San Jose CA and has $410 Million funding from Prime Movers Labs, Stellantis, FedEx, and Honeywell. https://lyten.com/products/batteries/ Celina Mikolajczak, a CalTech (Mechanical Engineering 1991) and Masters at Princeton graduate is Chief Battery Technology Officer at Lyten. She has worked at Panasonic Reno battery and QuantumScape.
Source:
QuantumScape August 2023 Shareholder presentation: https://s29.q4cdn.com/884415011/files/doc_presentation/2023/08/QS-IR-Presentation-August-23.pdf
Causes for LiON Battery Fires. What happens when batteries are abused: https://www.youtube.com/watch?v=VWMfeseybt4
QuantumScape Next-level Energy: The Real Benefits of Better Energy Density https://www.youtube.com/watch?v=DnYUUkECsro
QuantumScape 2020 Battery Showcase slide 23: https://s29.q4cdn.com/884415011/files/doc_presentation/2021/1/Data-Launch-Updated-Post-Presentation-20210107-2.pdf
January 27, 2022, QuantumScape white paper on battery charge speed: https://www.quantumscape.com/resources/blog/white-paper-a-deep-dive-into-quantumscapes-fast-charging-performance/
May 2022, Sila Nano white paper on their Silicon Anode drop in replacement for Carbon. https://www.silanano.com/uploads/Sila-_-A-Battery-Powered-American-Energy-Revolution-White-Paper.pdf
September 2, 2020, The Future of Energy Storage,Gene Berdichevsky, Gleb Yushin from Sila Nano https://www.silanano.com/uploads/Sila-_-The-Future-of-Energy-Storage-White-Paper-Updated-10.24.22.pdf
NASA Development of Solid-State Li/Sulfur- Selenium as Safe and High Capacity Battery (SABERS)The Minerals, Metals and Materials Society (TMS) 2020 149th Annual Meeting and Exhibition, San Diego, California, February 22-27, 2020 https://ntrs.nasa.gov/api/citations/20200001446/downloads/20200001446.pdf
CleanTechnica interviewed Celina Mikolajczak, Chief Battery Officer at Lyten, former Tesla cell quality engineer https://cleantechnica.com/2023/07/06/former-tesla-battery-expert-leading-lyten-into-new-lithium-sulfur-battery-era/
LG blog comparing NCM vs LFP battery chemistry: https://lghomebatteryblog.eu/en/this-is-why-ncm-is-the-preferable-cathode-material-for-li-ion-batteries/