Resilient and Confined Polymer Microlattices for Low-Swelling, Pressure-Free Lithium Metal Pouch Cells
Authors: Gao Yue , Chen Shu , Jolly Jason , Wang Shengfei , Wu Guanbin , Zhu Mengyao , Chen Tiantian , Han Chenyang , Wang Min , Ye Lei , Johnson Alissa Claire , Wang Bingjie , Yang Shu , Peng Huisheng , Pikul James H. 
Published: December 2024

Automotive and consumer applications require batteries that last longer, recharge faster, and readily integrate into mass-manufactured battery packs. Of the available cell packaging designs, pouch cells with lithium (Li) metal anodes are attractive because they can achieve ~95% packaging efficiency, allow size and shape flexibility, and have high theoretical specific energy. Li metal anodes, however, suffer from dramatic thickness increases as they are cycled, which leads to pouch cell swelling of up to 220% by volume. Although current Li metal pouch cells incorporate alternative electrolytes, protective layers, and current collectors to prevent Li dendrite growth and stabilize the solid-electrolyte interphase (SEI), high initial external pressures of 54 to 1,000 kPa are required to suppress cell swelling and maintain stable Li deposition. Maintaining a high pressure is impractical in most applications, requires considerable extra mass and volume not accounted for in reported battery metrics, and increases the complexity of cooling. Despite these applied pressures, Li pouch cells still swell at least 39%, which is far from commercial requirements of less than 10%, causes serious safety issues, and makes integration incompatible with most electric devices. In a cell with 39% swelling, the Li anode swells up to 300% so that the final anode volume is four times the initial volume and the volumetric capacity is 33% of Li’s 2093 mAh cm-3 theoretical capacity, which is lower than the volumetric capacity of lithiated graphite (790 mAh cm-3). Additionally, these challenges are escalated in high-capacity and high-rate Li metal pouch cells. New Li electrode designs that provide control over interfacial chemistry while accommodating the extreme mechanics of Li plating and etching are required to break current technical bottlenecks for practical Li metal batteries.

This talk will report the use of architected polymer microlattices that combine chemical and mechanical functions to enable high-rate, swelling-free, and pressure-free Li anodes and anode-less pouch cells. The 3D printed gyroidal polymer contains Li+ affinitive sites that accelerate Li+ transport and the gyroidal microgeometry is highly resilient and confines Li growth. Unlike prior Li anodes and porous Li scaffolds that need an SEI layer across large surface areas, 89% of the Li surface in the gyroid is covered by the polymer and the microgeometry is optimized so the remaining Li-electrolyte interface is protected by a LiF-rich SEI layer generated by F donating groups on the polymer. We study the LiF generated at the polymer and show that it moves up to 100 micrometers across the Li surface to enhance the anode stability. We also show how the combined chemical, mechanical, and geometric control allows dense and void-free Li metal to grow into complex 3D shapes. In a 5 mAh cm-2 pouch cell with zero external pressure, we demonstrate swelling-free Li deposition at 5-30 mA cm-2, or 1–6 C-rates, for over 100 cycles. Li deposition efficiency was 99.86% in a carbonate electrolyte with an areal capacity of 10 mAh cm-2 and 10 mA cm-2 current density. The Li anode achieved a 1,580 mAh cm-3 volumetric capacity. A 366 Wh kg-1 anode-less Li metal pouch cell (15.4 Ah) incorporating polymer microlattices on copper as the anode and a LiNi0.8Mn0.1Co0.1O2 (NMC) cathode achieved over 333 cycles with 80% capacity retention. The final cell swelling was below 2%. Insights into the excellent performance of these batteries can provide new paths towards achieving high rate and long life Li metal pouch cells with zero swelling.

Figure Caption: Polymer microlattice enabling pressure-free and swelling-free Li metal pouch cells. a, During Li deposition, the growth and merging of Li particles generate voids even under a pressure of 54-1,000 kPa, causing cell swelling of 39-220%. b, Polymer microlattices confine Li deposition in the gyroidal channels through chemical interactions and mechanical resilience. The polymer contains -NH2 or CH2CH2O- groups to accelerate Li+ transport and -SO2F to generate LiF to protect the remaining Li surface. The crosslinked polymer is highly resilient and restrains Li for dense deposition. c,d,Cycling stability (a) and voltage profiles (b) of an anode-less Li metal pouch cell, presenting a cell-level energy density of 366 Wh kg-1. NMC was used as the cathode. e, The measurement of cell thickness upon cycling. 6 sampling points were collected.

Figure 1