Fast Charging Formation of Lithium‐Ion Batteries Based on Real‐Time Negative Electrode Voltage Control

Abstract

In lithium-ion battery production, the formation of the solid electrolyte interphase (SEI) is one of the longest process steps. [1] The formation process needs to be better understood and significantly shortened to produce cheaper batteries. [2] The electrolyte reduction during the first charging forms the SEI at the negative electrodes. [3,4] Besides that, a SEI is also formed at the positive electrode (PE-SEI) during the first cycles. [5,6] Especially, the SEI has a substantial impact on the battery's performance and aging by limiting further reductive decomposition of the electrolyte. [7] The SEI material compositions and electrochemical properties have been reviewed extensively. [7][8][9][10] State-of-the-art formation procedures in the industry last up to multiple days [1] and consist of several charge and discharge cycles. [11] Long periods at high state of charges (SOCs) increase the reduction rate at the negative electrode (NE) and the oxidation rate at the positive electrode (PE) that lead to the formation of the SEI and PE-SEI, respectively. [12] Higher SOCs lead to lower NE potentials which exponentially increase the SEI growth rate whereas increased C-rates have a linear relation. [13] Similar results for accelerated SEI growth at low NE potentials were simulated by a model based on differential voltage analysis. [14] Formation strategies with multiple subcycles at high SOCs are recommended in the literature as it was found to guarantee stable surface layer properties that resulted in good cell performance while ensuring shorter formation times, e.g., 21, [15] 14, [1] and 13 h. [16] Increased ambient temperatures and external pressure enabled a formation time about 3 h. [17] Variable fast charging current rates based on an electrode equivalent circuit model even resulted in a formation time below 1 h without negative impacts on cell performance compared to longer reference formations. [18] However, comparing formation times between different cell configurations is complicated, as different materials (e.g., electrolyte or active material) or properties (e.g., coating thickness or porosity) influence the maximum applicable current. [19] Charging currents that lead to negative NE potentials may form lithium-plating on the NE's surface [20][21][22] as lithium ions react to metallic lithium depositions instead of intercalating into the NE. [23,24] In general, lithium-plating is an undesired sidereaction which comes along with capacity loss and may result in an internal short circuit due to dendrite formation. [25][26][27] Just a part of the plated lithium is reversible and reacts back to lithium ions during discharging, which is called lithiumstripping. [28] Irreversible lithium-plating can be proved by cell disassembly and optical investigation of the NE. Therefore, it is recommended to discharge the cell to 0% SOC followed by a

Transient Self-Discharge after Formation in Lithium-Ion Cells: Impact of State-of-Charge and Anode Overhang

“…57 Drees et al investigated five formation procedures with varying cycle times and found no significant quality differences between fast and slow formation as long as lithium plating was prevented. 26 These results contrast formation studies with extended periods in the high voltage range during formation, where improved cell capacity retention was reported. Weng et al reported a small but highly reproducible positive impact of fast formation in the high voltage range on the cycle life of 2.36 Ah stacked pouch cells.…”

“…Even though the effect of the anode overhang is already well documented and present in most commercial cell designs, it is often neglected in the evaluation of formation and aging studies. 2,[22][23][24][25][26] Typically, the geometric anode overhang of commercial cells is between 3% to 10%. 5,10,27,28 However, some formation studies were performed with an anode overhang of more than 20%.…”

Lithium-ion battery cell formation: status and future directions towards a knowledge-based process design

“…Although most of the research on the formation process focuses on the interfacial layers and the 33,[147][148][149] (C) Fast single-cycle formation with high currents at low NE potentials. 27,135,149,153 Reprinted with permission from Drees et al, 149 John Wiley and Sons, Copyright 2022. decomposition of the electrolytes, the degradation effects of the active material particles have also been investigated. Cheong et al adopted different current densities in the initial formation cycle and observed how the morphological evolution takes place for Co 3 O 4 nanoparticles as shown in Fig.…”

“…17 Drees et al compared the protocol of An et al 148 with another sub-cycling formation protocol. 149 The new protocol used 0.8C and removed the constant voltage (CV) phases during the sub-cycles, resulting in a reduced formation time of only 10 h, without significant impact on the cell performance. Thus, multiple studies show that formation time can be reduced without significantly compromising the cell quality.…”

“…(B) Multi-sub-cycle formation with periodic sub-cycles at high SOC (low NE potentials) 33,[147][148][149]. (C) Fast single-cycle formation with high currents at low NE potentials 27,135,149,153. Reprinted with permission from Drees et al,149 John Wiley and Sons, Copyright 2022.…”