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The Most Promising Routes to a High Li Transference Number Electrolyte for Lithium Ion Batteries Kyle M Diederichsen, Eric J McShane, and Bryan D. McCloskey ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.7b00792 • Publication Date (Web): 06 Oct 2017 Downloaded from http://pubs.acs.org on October 7, 2017
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ACS Energy Letters
Abstract
The continued search for routes to improve the power and energy density of lithium batteries for electric vehicles and consumer electronics has resulted in significant innovation in all cell components, particularly in electrode materials design. In this perspective, we highlight an often less noted route to improving energy density: increasing the Li+ transference number of the electrolyte. Turning to Newman’s original lithium ion battery models, we demonstrate that electrolytes with modestly higher Li+ transference numbers than traditional carbonate-based liquid electrolytes would allow higher power densities and enable faster charging (e.g., >2C), even if their conductivity was substantially lower than conventional electrolytes. Most current research in high transference number electrolytes (HTNE’s) focuses on ceramic electrolytes, polymer electrolytes, and ionomer membranes filled with nonaqueous solvents. We highlight a number of the challenges limiting current HTNE systems, and suggest additional work on promising new HTNE systems, such as ‘solvent-in-salt’ electrolytes, perfluorinated solvent electrolytes, nonaqueous polyelectrolyte solutions, and solutions containing anion-decorated nanoparticles.
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Table of Contents Image
For Table of Contents Use Only
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Consumer Li-ion battery powered devices are ubiquitous in modern society and significant growth in the electrification of vehicles, powered by advances in Li-ion batteries, has occurred in the last several years. Plug-in hybrid (PHEV) and pure electric vehicles (EVs) accounted for 0.9% of total cars purchased globally in 2016, and sales of all plug-in vehicles increased 550% from 2012 to 2016, with nearly 780,000 PHEV/EV sales (160,000 in the United States) in 2016.1 However, current consumer electronics and EV’s still suffer from a few major challenges related to their batteries: they are larger and heavier than desired, they take a long time to charge, and safety incidents continue to attract significant media attention. For Li-ion batteries to continue their impressive market penetration over the next few decades, breakthroughs in battery technology are required that continue to directly address these issues. For this reason, significant research has targeted increased energy density, higher charge and discharge rate, and improved safety through various advances in every component of the Li-ion battery.2,3 Electrolyte mixtures and separators have been identified that enable reasonably safe, long term use,4,5 and cell designs and energy management technologies have enabled impressive gains in cycle life, safety, and energy density.6 Recent work has particularly emphasized improving the energy density of Li-ion electrode materials. As relevant examples, Li alloys (Si and Sn being the most well-studied) or pure Li metal are being explored as high-capacity anode materials, and the so-called Li-rich transition metal oxides are being pursued as cathode materials.7–11 The focus of this perspective is another, sometimes overlooked, strategy to improve energy density and charging rates of batteries, namely the development of high Li+ transference number electrolytes (HTNE’s), those in which the ionic current is carried predominantly by the Li+ rather than its counteranion. The definition of the Li+ transference number in the dilute limit for a
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binary salt electrolyte in which both ions are univalent (a 1:1 electrolyte) relates the diffusion of Li+ and its counterion through the following simple relationship: =
+
(1)
Where t+ is the Li+ transference number, D+ is the Li+ diffusion coefficient, and D- is the anion diffusion coefficient.12 In this limit the transference number can be considered as simply the fraction of the total ionic conductivity that is carried by Li+. Despite numerous advances in porous electrode materials, improvement of electrolyte properties, including stability, ionic conductivity, , and t+, remains one of the most important challenges to any further performance gains. Conventional electrolytes are based on a binary lithium salt and stabilizing additives dissolved in a mixture of carbonate liquid solvents.4 These electrolytes are then imbibed in the porous polyethylene or polypropylene separator and electrodes to create ionic contact while maintaining electronic insulation between the electrodes. This class of electrolytes imparts high lithium-ion conductivity and has been successfully commercialized for many years, but suffers from a couple of key drawbacks; we will briefly discuss the first drawback prior to focusing the remainder of this perspective on the second. First, all liquid electrolytes possess a voltage stability window, typically between ~1 V and ~4.5 V vs Li/Li+, that ultimately limits battery rechargeability, safety, and high-energy active electrode materials development.13 To address capacity fade during battery cycling, in combination with limiting the upper operating voltage to improve stability at the cathodeelectrolyte interface, carbonate-based electrolytes typically contain small amounts of organic additives which are usually selected through empirical combinatorial analyses, although effort has been made to develop a deeper understanding of the topic.5,14 During the first few dischargecharge cycles, these additives will facilitate the formation of solid, ion-conducting layers on the
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ACS Energy Letters
anode surface that otherwise substantially suppress parasitic reactions between the anode and electrolyte.15 While commercial mixtures of additives have been realized after many years of research, understanding the influence of additives on electrode processes at a molecular level remains a poorly understood topic. Second, although current liquid electrolytes offer high conductivity across a wide temperature range (~1-10 mS/cm) and well dissociated ions with a solubility >1 M Li+, they have a Li+ transference number below 0.5, indicating the majority of the total ionic conductivity is in fact the result of anion motion. This low t+ occurs due to the strong preferential solvation of Li+ over its counterion, resulting in a bulky solvation shell around Li+ compared to that of typical anions.16 Within a lithium ion battery, anions tend to migrate in the opposite direction of the lithium and eventually accumulate at the electrode surface, resulting in the buildup of a concentration gradient, as is shown in the Table of Contents Scheme. This concentration gradient limits the rate at which the battery may be charged or discharged, creates a concentration overpotential that limits the operating voltage of the cell, and limits the thickness of electrodes that may be used, all of which limit the power and energy density of the cell. All of these challenges could be substantially alleviated if a high t+, high electrolyte were developed. However, as will be discussed below, the creation of organic electrolytes that provide both high t+ and has been elusive due to the inherent limitations of ion transport in systems where the anion is immobilized. Ceramic single ion conductors (SIC’s), such as garnet phase Li7La3Zr2O12,17 have been shown to provide high conductivity with unity transference numbers, but are limited for practical use by their brittle nature when configured in thin form factors, among other challenges. In this perspective, we specifically highlight the challenges facing, and strategies for, the development of practical high Li+ transference number electrolytes, as well as
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the importance of t+ on ultimate attainable energy and power density in Li-ion batteries. Included in our discussion is the vexing problem of precisely measuring the Li+ transference number, which is in fact a non-trivial and debated issue. We begin by quantifying the benefit that may be expected from increasing t+ in Li-ion electrolytes.
The importance of a high transference number electrolyte. Doyle, Fuller, and Newman in 1994 demonstrated the importance of the lithium ion transference number, showing that a t+ of 1 offers significant enhancement in terms of materials utilization, power, and energy density over a t+~0.2, particularly at high rates of discharge and even with an order of magnitude decrease in conductivity.18 It is shown here that even modest improvements in t+, e.g., to t+ ~0.7, would be beneficial, particularly allowing a higher attainable state of charge at high charge rates, where a large, constant current would be necessary to quickly charge the battery. To understand useful targets for t+ and to enable fast charging, we turn to finite element methods based on Newman’s 1D isothermal battery model. Using the Batteries and Fuel Cells Module in COMSOL Multiphysics 5.2a, we model charging of a dual lithium ion insertion cell consisting of a porous graphite anode, a porous LiCoO2 cathode, and an electrolyte of varying transport properties, with a simple schematic of the cell shown in Figure 1A. We study charge instead of discharge because EV batteries are discharged at high rates only intermittently (during acceleration), such that the salt concentration gradients that ultimately limit cell performance do not evolve to the extremes that would be expected during high rate charging. The parameters used are based on realistic values of state-of-the-art cell compositions and are given in
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Supporting Information Table 1. The full model is more completely discussed in the Supporting Information. The goal of modeling this cell configuration is to compare high t+ electrolytes (where both t+ and σ are varied) to a standard binary salt liquid electrolyte, for which t+~0.4 and is 10 mS/cm, where all other battery properties (including SEI stability and impedance) are similar. Given many studies that attempt to improve electrolyte t+, one would expect an inherent tradeoff between and t+, so we chose to study charge behavior of cells where t+ of the electrolyte is varied between 0.4 and 1, and is varied between 1 and 10 mS/cm (i.e., higher t+ and lower
than traditional liquid electrolytes). Typical charge profiles (with a voltage cutoff of 4.2V) are shown in Figure 1B, and the total attainable state of charge (SOC) prior to hitting the cutoff voltage is shown as a function of charge rate for these cells in Figure 1C and 1D. Although little difference is observed in the attainable SOC at low current densities (
