1. Scientist – Polaris Battery Labs
Strategies and Roadblocks to Commercialization of High Capacity Lithium Ion Batteries
Nicholas Gurnon
Scientist – Polaris Battery Labs

2. Lithium Ion Battery Prototyping at Polaris
The Polaris Team
Artisanal Batteries
Highly customizable
Hand-made
Electrode raw materials
Separator
Intro to me and to Polaris. Clients include Entrepreneurs, Universities, National Labs, Startups, Established companies in other industries trying to break into battery space, Large consumer product companies. We get to work with materials that may be the next big thing in lithium ion batteries. Photos: Lab, coworkers, building, cell formats, raw materials - powders, solvents, foil, separator, pouch, electrolyte, tabs.
Current collectors (metal foil)
Wide variety of cell formats

3. Separator 3862 mAh/g 372 mAh/g 175 mAh/g ~1960 mAh 508 mg Li ~2940 mAh
Composite Cathode Material for Li-Ion Batteries Based on LiFePO4 System. DOI: /21635
Low Potential
Full charge ~ 0.01 V vs. Li/Li+
High Potential
Full charge ~ 4.3 V vs. Li/Li+
Separator
3862 mAh/g
372 mAh/g
175 mAh/g
~1960 mAh
508 mg Li
~2940 mAh
761 mg Li
Overview of LIB function. Difference between primary (lithium battery, non-rechargeable) and secondary (lithium ion battery, rechargeable) cell:major difference is that bulk lithium metal does not actually exist in a properly functioning lithium ion battery (although it did in the Galaxy 7 battery). Lithium metal in a rechargeable cell causes all kinds of problems, from rapid oxidation events due to short circuit to severe capacity fade. During charge, lithium atoms in the cathode are stripped of an electron, diffuse through the electrolyte to the anode where they are reunited with an electron, and the process is reversed on discharge. Drawback is that the cathode has a capacity of ~160 mAh/g, usually contains expensive and heavy transition metal oxides, while lithium metal is very light and has a capacity of 3862 mAh/g.
Graphics: Lithium metal disc. Li+ and e- leaving one electrode and heading to other, Define mAh – unit of capacity equal to a current of 1 mA, sustained for 1 hour. 1 mA = 1 mC/s = x 1015 electrons/Lithium ions passing through a given point per second. iPhone 7 has battery capacity of 1960 mAh, 7 plus has ~50% more
1mA = 1 𝑚𝐶 𝑠 = x 1015 elementary charges passing through a given point per second
1mAh = unit of capacity equal to a current of 1 mA, sustained for 1 hour = x 1019 elementary charges = 0.26 mg lithium

4. Solid Electrolyte Interphase (SEI):
Complex, Heterogeneous, Difficult to Study, Magical
Image Source: MIT Technology Review Website
2Li + H2O → 2LiOH + H2 + heat = caustic mess + FIRE
Focus on Electrolyte: it is asked to maintain certain physical qualities throughout a pretty large temperature range (i.e. stay liquid with low viscosity, maintain solubility of a lithium containing salt in order to remain ionically conductive), and also to remain stable over a wide voltage window (0 – 4.3 V vs. Li/Li+). Aqueous electrolytes won’t work because water undergoes oxidation/reduction within this range, and, by the way, it reacts violently with lithium. Many combinations of organic solvents have been tested, and it turns out that none exactly meet the voltage stability requirement, particularly on the low voltage end. HOWEVER, it was found that certain compounds would reductively decompose at the anode (low voltage) surface to form an electrically insulating but ionically permeable membrane, called the solid electrolyte interphase, that impedes further reductive decomposition of the electrolyte. Graphics: standard electrolyte components, decomposition products of electrolyte *Note that lithium is consumed in the formation of the SEI layer*
Image Source: Webb Group website, University of Texas at Austin

5. Not All Anode Materials Created Equal
Fully Charged Graphite: LiC6
~372 mAh/g
Fully Charged Silicon: Li22Si5
~4200 mAh/g
Intercalation
Alloying
Graphite vs. Silicon: Intercallation vs. Alloying and capacity (stoichiometry) of the fully lithiated anode. Graphite is electrically conductive and is composed of parallel, offset sheets of graphene, into which lithium ions can diffuse and find a comfy home in the center of a benzene ring to reunite with their electrons, separate from other lithium atoms (i.e no bulk lithium metal is formed). Fully lithiated graphite has the formula C6Li – one lithium atom for every 6 carbons. That works out to be about 370 mAh/g of graphite. On the other hand, Silicon, which forms an alloy with lithium, has a fully lithiated stoichiometry of Li22Si5, or Li4.4Si – more than 4 lithium atoms per Si mAh/g of silicon. What does that mean? It would seem that it might mean over 2x the battery (after all, the anode is only one of the cell components) in the same size package. So why isn’t this everywhere? (silicon is found in some commercial cells but only as a minor component in a majority graphite anode, giving a ~25% reversible capacity boost to the anode). Graphics: lithium intercallation in graphite, Li-Si stoichiometry, column graph of capacity

6. All of That Lithium Has to Go Somewhere
Mater. Chem. Front., 2017, Advance Article (DOI: /C6QM00302H)  Si
Li4.4Si C
Li C6
Charge
Discharge
Lithiated Graphite:
10% Volume Expansion
Lithiated Silicon:
300% Volume Expansion
SEI
Particle cracking, continuous SEI growth, loss of electrical connection, unstable anode voltage lead to rapid capacity fade
Mostly, it goes back to the SEI layer, and the fact that you can’t just cram something into something else and expect the volume not to change. Non-instantaneous, non-uniform diffusion into silicon means that there will be differential expansion of the silicon particles, with the outside of the particle expanding quicker than the inside: cracking, pulverization, exposure of fresh silicon surface to electrolyte, new SEI formed (lithium consumed), separation of silicon material from bulk (no more electrical connection, becomes floating garbage). In similarly destructive fashion, the reverse happens on discharge/delithiation: differential contraction, cracking, pulverization, etc. All of this means lost lithium, or lost capacity. Graphics: illustration of 10% vs. 300% expansion (spheres), photos of pristine and cycled graphite and silicon, plots of capacity fade for graphite and silicon based cells using typical CEs (99.9% for graphite, 97% for silicon cycled over whole voltage range)

7. 10x surface area means at least 10x SEI per gram of anode material
Graphite - 20 µm diameter
Surface area ~2 m2 per gram
Use Smaller Particles
Less severe differential expansion = decreased internal stress = reduced cracking
10x surface area means at least 10x SEI per gram of anode material
Chem. Commun., 2012,48,
Silicon - 50 nm diameter
Surface area ~20 m2 per gram
Particle size and binder. Smaller particles mean relatively less strain between outer surface (rapidly expanding) and inner core, which should translate into less cracking = less new exposed silicon surface area = less new SEI formation. HOWEVER, particles have to get VERY small in order for this to make a significant difference, on the order of nm, and that means significantly more surface area exposed to electrolyte, which means more SEI formation, which consumes lithium from the system. Also, silicon pretty quickly forms a thin oxide layer on the surface when exposed to air, and silicon oxides will react with the electrolyte components to form additional decomposition products, consuming lithium in the process. Also also, unlike graphite silicon is a pretty poor conductor, so you need to make sure all of the particles are electrically connected through the electrode coating and down to the current collector which requires a lot of conductive additives, which increases the total surface area without adding capacity. Interestingly, a lot of progress has been made with perhaps the least exciting (at least on the surface) component of the anode: the binder (glue). By strategically selecting the polymer binder, or using more than one type, researchers have been able to use the glue as an “artificial” SEI, blocking electron transport from the anode to the electrolyte (i.e. reductive decomposition) while still allowing ionic transport. The result is reduced capacity loss associated with the initial formation of SEI during the first charge/discharge cycle. Additionally, manipulating the pH of the slurry to affect the nature of the binder-silicon bond can significantly increase cycle life by decreasing the incidence of particle fracture and/or dislocation from the conductive matrix. Graphics: 20 um graphite particle next to 50 nm silicon particle, label with surface area per gram of each material and ICL numbers: 30 mAh/g for graphite vs mAh/g for silicon (combination of particle surface area and cracking that reveals more surface area), binder interaction with Si particle schematic, cycle life with pH adjusted slurry.

8. Build a Better SEI with Electrolyte Additives
Chem. Commun., 2012,48,
Electrolyte Additives. Returning to the electrolyte, decomposition products from the electrolyte make up the SEI layer, right? So what if we could put additional chemicals in the electrolyte that would decompose to form a BETTER SEI – more ionically conductive, more uniform, more resilient and better able to expand/contract without rupturing as badly. Original breakthrough for silicon was the addition of FEC, which decomposes at a higher voltage (lower SOC) than ethylene carbonate and forms a “better” SEI layer. FEC does have limitations, including temperature range sensitivity and the potential to degrade and release HF into the cell. Still, it’s pretty much guaranteed that any cell with silicon in the anode, commercial or research, will contain FEC and/or other electrolyte additives that function in a similar way. Many other electrolyte additives have been tested and combinations of them are very promising. Problem is that the evaluation takes a lot of time; have to test the electrolyte under realistic conditions over hundreds of cycles in order to get a truly accurate picture of what’s happening. Complicating this task is the much less well understood matter of oxidative decomposition that occurs on the cathode surface, the products of which can travel to the anode and affect the qualities of the SEI. Graphics: cycle life with and without FEC, FEC and EC molecular structure, voltage of reductive decomposition for EC and FEC

9. Limit Expansion/Contraction by Controlling State of Charge (SOC)
Li1.2Si
Partial Charge
~100% Expansion
Voltage ~ 0.15 V
Full Discharge
Full Contraction
Voltage ~ 1.5 V
Journal of The Electrochemical Society, 163 (6) A1020-A1026 (2016)
Anode capacity ~1200 mAh/g silicon
Moderate ICL – comparable cell capacity to graphite
Lower Cell Voltage than graphite
Reduced volumetric energy density
Large ICL - lower cell capacity than graphite
Comparable Cell Voltage to graphite Si
Li4.4Si
Li3.2Si
Full Charge
Partial Discharge
300% Expansion
Voltage ~ 0.01 V
Partial Contraction
Voltage ~ 0.4 V
Control expansion and contraction by limiting the state of charge/lithiation of the silicon. This works pretty well actually, and you still get ~3-4x the gravimetric capacity of graphite. HOWEVER, state of charge is proportional to voltage, so if you stay in the low state of charge regime, limiting the extent of expansion, you get the benefits of lower initial capacity loss and low fade, but your overall cell voltage is relatively low (e.g. nominal 3.2 V vs. 3.7 V for equivalent cell with graphite anode). If you choose to stay in the high state of charge region, i.e. full expansion but limited contraction, you have to pay a very large penalty in the ICL, wiping out more than half of the capacity of your cathode in the first cycle. State of charge/voltage limitation is a very important strategy that will work in specific applications, however. Graphic: silicon primary cell cycling flat at ~1200 mAh/g, energy density (Wh/L) equation and typical graphite cell vs capacity limited silicon cell
Cell Voltage (𝑉) = 𝑉 𝐶𝑎𝑡ℎ𝑜𝑑𝑒 − 𝑉 𝑎𝑛𝑜𝑑𝑒
Volumetric Energy Density ( 𝑊ℎ 𝐿 ) = 𝐶𝑎𝑝𝑎𝑐𝑖𝑡𝑦 𝐴 𝑥 𝑁𝑜𝑚𝑖𝑛𝑎𝑙 𝑉𝑜𝑙𝑡𝑎𝑔𝑒 (𝑉) 𝐶𝑒𝑙𝑙 𝑉𝑜𝑙𝑢𝑚𝑒

10. Engineered Silicon Morphologies People are Creative!
Encapsulated Silicon
Silicon Nanowires (Amprius)
Nano Lett., 2012, 12 (6), pp 3315–3321
Nature Nanotechnology 3, (2008) 
Graphene-coated Silicon Nanoparticles
Scientific Reports 4, Article number: 3863 (2014)
Other silicon anode morphologies and approaches: people are creative! egg yolk structures that allow for expansion and contraction within a conductive graphite shell, surface functionalization of silicon particles to increase binding affinity and reduce dislocation from conductive network, decrease contact resistance and overall effective surface area, modification of binders to cover surface and act as artificial SEI, and Nanowires allow for high surface area and, being essentially “1 dimensional” and grown by vapor deposition, linear electrical connectivity to the current collector (commercialization is now a reality thanks to Amprius, who has used this technology to achieve a 20% gain in volumetric energy density over the best graphite based lithium ion cells: Graphics: egg yolk structure, graphene wrapped structure, schematic of organic groups on silicon surface, nanowires

11. Thank You! Want to talk batteries? ngurnon@polarisbatterylabs.com