How Observable Is Lithium Plating? Differential Voltage Analysis to Identify and Quantify Lithium Plating Following Fast Charging of Cold Lithium-Ion Batteries

Abstract

Fast charging of batteries is currently limited, particularly at low temperatures, due to difficulties in understanding lithium plating. Accurate, online quantification of lithium plating increases safety, enables charging at speeds closer to the electrochemical limit and accelerates charge profile development. This work uses different cell cooling strategies to expose how voltage plateaus arising from cell self-heating and concentration gradients during fast charging can falsely indicate plating, contrary to prevalent current assumptions. A solution is provided using Differential Voltage (DV) analysis, which confirms that lithium stripping is observable. However, scanning electron microscopy and energy-dispersive X-ray analysis are used to demonstrate the inability of the plateau technique to detect plating under certain conditions. The work highlights error in conventional plating quantification that leads to the dangerous underestimation of plated amounts. A novel method of using voltage plateau end-point gradients is proposed to extend the sensitivity of the technique, enabling measurement of lower levels of lithium stripping and plating. The results are especially relevant to automotive OEMs and engineers wishing to expand their online and offline tools for fast charging algorithm development, charge management and state-of-health diagnostics. Fast charging of lithium-ion batteries remains a priority amongst automotive Original Equipment Manufacturers (OEMs) that are electrifying their product portfolios. Consequently, there are increasingly large demands on time and monetary investment in the development of fast charging strategies, for which, the avoidance of lithium plating is a priority. However, the techniques available for inexpensive, non-destructive and fast plating identification and quantification are immature and contribute to the cost of development. Moreover, increased accuracy of plating assessments is required for improved online, Battery Management System (BMS)-based diagnostics and charge strategy de-rating. Researchers also demand alternatives to destructive evaluation techniques. The instability of Li 0 coupled with the timeframe between plating and cell disassemly presents a major problem because ex-situ observations may no longer be accurate. 1 During charging of a lithium-ion cell with a graphitic Negative Electrode (NE) the reversible lithium intercalation reaction proceeds in the reduction direction according to Equation 1 at the NE. Under certain conditions, overpotential exceeding NE equilibrium potential or Li concentration saturation of the graphite, the competing lithium metal deposition reaction additionally proceeds. It does so in the reduction direction according to Equation 2, inducing lithium plating (deposition). Li x C 6 oxidation − −−− → ← −−− − reduction xLi + + xe − + 6C [1] Li 0 oxidation − −−− → ← −−− − reduction (1 − x)Li + + (1 − x)e − [2] Since both reactions are reversible, cell discharging oxidizes lithium sourced from both reactants, Li x C 6 and Li 0. Versus Li x C 6 oxidation, the relatively facile Li 0 oxidation (stripping) process and associated lower oxidation potential produce a high voltage discharge plateau. 2,3 The plateau has traditionally been used to identify Li 0 stripping , and by extension, lithium plating. 4-7 Extensions of those works report the discharge plateau measurement as a technique for the semi-quantitative assessment of lithium plating. 2,8 Petzl & Danzer further reported the technique as fully quantitative, employing differential voltage analysis for unambiguous determination of plateau length. 9,10 In this work, the phrase "the technique" is used to refer to the practice of using the high voltage plateau to identify and/or quantify lithium plating and stripping. Throughout the technique's evolution from a qualitative to a quantitative tool, few works have addressed its accuracy and reliability or considered in full the difference between quantifying stripping and quantifying plating. 11 Moreover, the use of the technique for lithium plating identification alone is still not fully understood and is without consensus. The traditional belief that an absence of a plateau is indicative of an absence of plating 2,5 has been challenged, but remains unclear. The concern by Smart et al. 5 that Chemical Intercalation (CI) could result in a reduction or absence of Li 0 availability, and consequently the absence of a stripping plateau in spite of plating having occurred, has been reiterated. 12-15 It has been suggested that the stripping current is dependent upon the areal coverage of lithium 16 and additionally, that the area be of a minimum size for detection. 11,14 Following plating, the interplay between CI, stripping and irreversible Loss of Lithium Inventory (LLI) owing to parasitic reaction is acknowledged as complex , and requires further studies to develop the state of knowledge. The technique has been nearly universally developed and studied i) under the conditions of slow charging where cell self-heating is insubstantial and with cell temperature deviating little spatially and versus the ambient, and additionally, where solid-phase lithium concentration gradients are small, or ii) at ambient temperatures so low that they are rarely applicable to Electric Vehicles (EVs). This development history has led to perpetuation of the theory that the technique functions only at temperatures of −20 • C and below. 13,14,16 However, the discharge plateau technique's performance is arguably of greatest interest to the automotive sector following fast charging, when the propensity to lithium plate is greater than following slow charging, and at temperatures more commonly encountered by EVs. A dearth of information exists under these conditions, as Table I demonstrates. The majority of works given in Table I report only ambient temperatures and neglect the difference relative to cell temperature. Uhlmann, Illig, Ender, Schuster & Ivers-Tiffée investigated plating following 10 C charge pulses at 23 • C, but via the alternative relaxation technique. 16 While works such as that by Tippmann, Walper, Balboa, Spier & Bessler have simulated the influence of cell self-heating on NE potential during charging, few have done so with a focus on the high voltage plateau. 11,17 Yang, Ge, Liu, Leng, & Wang probed via simulation the influence of temperature on the voltage curves, again during relaxation, for the same quantity of Li 0. 11 Similarly, little attention has been afforded to the influence of instantaneous State of Charge (iSOC)) unless CC License in place (see abstract). ecsdl.org/site/terms_use address. Redistribution subject to ECS terms of use (see 141.212.177.21 Downloaded on 2019-05-14 to IP