Elucidating the Effect of Pre-Lithiation of Silicon Dominant Anodes on the Full Cell Performance
Authors: Bock Moritz , Dickmanns Jonas L. S. , Gasteiger Hubert Andreas 
Published: August 2024

Over the last few years, there has been a high interest in increasing the energy density of lithium-ion batteries in order to increase the driving range of battery electric vehicles. On a material level, silicon is a promising anode active material candidate due to its high theoretical capacity of 3579 mAh g-1, which is approximately ten-fold higher than that of the currently used graphite anodes with a theoretical capacity of 372 mAh g-1.[1] However, the main drawbacks of silicon are its intrinsic volume expansion of +300 % upon lithiation.[2] This leads to rupturing of the passivating solid electrolyte interphase (SEI), particle cracking, and loss of electronic contact, limiting the lifetime of batteries with silicon based anodes, thus hindering their transition into application.

As a possible solution, Jantke et al. proposed the concept of partial utilization of µm-sized crystalline silicon, where only one-third of the theoretical capacity is used for (de)lithiation. Thereby, a crystalline core remains, and the volume expansion on the particle-level is reduced to approximately + 100%.[3] In addition, it was shown that irreversible losses during the formation and upon cycling can be compensated by pre-lithiation of the anode, which increases the lifetime significantly.[4,5]

In this study, we investigated a promising cell chemistry for high-energy applications, containing a Si-dominant anode and a cathode based on a Co-free lithium- and manganese-rich NMC. The here used anode consists of 70 wt% silicon, 20 wt% graphite, 2 wt% conductive C65, and 8 wt% LiPAA-binder. Note that graphite mainly acts as a conductive additive within the operating voltage window, with a negligible contribution to the overall anode capacity. The electrochemical testing was conducted at 25 °C in Swagelok® T-cells containing two glass fiber separators, a 1M LiPF6 in FEC:DEC (2:8, v:v) electrolyte and a lithium metal reference.

Without pre-lithiation, these full-cells suffered from rapid capacity fading, reaching the 80 % state-of-health (SOH) criterion after ~100 cycles. As will be discussed, our analysis shows that the primary mode of failure in these cells is the loss of cyclable lithium, evidenced by an increase in the end-of-discharge potential at the anode. It was found that with increasing end-of-discharge potential at the anode, the anode resistance at low state-of-charge rises significantly.

Therefore, silicon anodes were lithiated in coin half-cells to 25, 50, and 75 % of the reversible cathode capacity (corresponding to 300, 600, and 900 mAh gSi
-1). Then, the anodes were harvested in the lithiated state and assembled in a Swagelok® T-cell with the same cathode, separator, and electrolyte as the non-pre-lithiated reference cells. Pre-lithiation leads to similar reversible charge/discharge capacities, but generates a cyclable lithium reservoir in the anode.

Successively increasing the pre-lithiation of the anode increases the lifetime from 100 to 550 cycles. The onset of the above-described aging phenomena is delayed by 150 cycles for every additional 25 % pre-lithiation. Further, the influence of aging and lithium inventory on anode and cathode resistance will be discussed, showing that the increase in anode resistance is mainly a function of (de)lithiation rather than cycle number.

Acknowledgments:

The authors gratefully acknowledge the funding from the BMBF (Federal Ministry of Education and Research, Germany) in the ExZellTUM III project (grant number 03XP0255). We also kindly thank Wacker Chemie AG and BASF SE for providing the anode and cathode active materials.

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