Negative electrodes in rechargeable lithium ion batteries — Influence of graphite surface modification on the formation of the solid electrolyte interphase

Abstract

Abstract.Rechargeable lithium ion cells operate at voltages of ~ 4.5 V, which is far beyond the thermodynamic stability window of the battery electrolyte. Strong electrolyte reduction and corrosion of the negative electrode has to be anticipated, which leads to irreversible loss of electroactive material and electrolyte, and thus strongly deteriorates cell performance. To minimize these reactions, negative electrode and electrolyte components have to be combined bringing about the electrolyte reduction products to form an effectively protecting film at the anode/electrolyte interface. This film hinders further electrolyte decomposition reactions and acts as membrane for the lithium cations, i.e., behaves as a solid electrolyte interphase (SEI). The present paper gives a review of our recent work in the field of negative electrodes in lithium ion batteries. The effects of the graphite anode surface and graphite anode surface modification on the formation of the SEI are discussed in detail by using the example: modification with carbon dioxide. 1, Introduction 1.1. Lithit,n Ion Batteries. The present development of lithium ion cells can be considered to be mainly concerned with the serious problems related to the cycling (= periodic dissolution and deposition) of metallic lithium negative electrodes (anodes) in rechargeable batteries.Lithium plating occurs more or less dendritically and in side reactions lithium corrosion and passivation takes place. The dendrites are covered with a passivating film and become therefore partly electrochemically inactive, which has to be compensated by a large excess of lithium. Moreover, the dendrites may form filaments which locally times that of LiC6) and have therefore been repeatedly suggested as anode materials for lithium ion batteries [4]. Unfortunately, the uptake and release of Li is accompanied by enormous volume changes (e.g., from Sn to Li22Sns: approximately 250 % volume increase; by comparison, from graphite to LiC6: only approximately 10 % volume increase), which in the case of "ordinary" coarse-grained, bulky metal host materials leads to cracking and crumbling of the electrode and hence renders an application in rechargeable batteries impossible. In recent years it has been found that only specially designed nano-structured multiphase metal hosts allow for long term "full capacity" charge/discharge cycling (for further information the reader is referred to the reviews [2][3][4]). However, all measures applied so far to improve the cycling stability of metal hosts have not yet resulted in a commercially available anode material. Carbons and in particular graphites are still the first choice anode materials. The Solid Electrolyte Interphase. Lithium ion cellsexhibit cell voltages of up to 4.5 V and therefore operate far beyond the thermodynamic stability window of the typically organic solvent-based electrolytes. Electrolyte decomposition occurs at both electrodes. Fortunately, electrolyte reduction products, created in situ during charge, form a protecting film at t...

Gas Evolution at Graphite Anodes Depending on Electrolyte Water Content and SEI Quality Studied by On-Line Electrochemical Mass Spectrometry

“…The evolution of 2000 ppm C 2 H 4 in our cell setup corresponds to an absolute amount of 0.78 μmol of C 2 H 4 (using the cell volume of 9.5 ml and the molar gas vol- ume). Considering the electrolyte volume 120 μl LP57 at a density of 1.2 g/cm 3 and the EC to EMC ratio of 3/7 wt/wt, the amount of EC in the cell amounts to 43.2 mg. With the molecular weight of EC (88 g/mol) that number translates into 419 μmol of EC. Assuming that two reduced EC molecules recombine to release one C 2 H 4 molecule, 12 we can conclude that 2 · 0.75 μmol = 1.50 μmol of EC were reduced during the first negative going scan.…”

“…The OEMS test cells work with an electrolyte excess of (120 μl · 1.2 g/cm 3 ) / (5.8 mg/cm 2 · 1.767 cm 2 ) = 14/1 g electrolyte /g graphite . In a typical battery the weight ratio of electrolyte to active material is ≈0.35/1 g electrolyte /g AM according to Wagner et al 38 In a typical graphite/NMC cell with a balancing of 1.1/1 in units of mAh/cm 2 of the graphite electrode versus mAh/cm 2 of the NMC electrode, and theoretical capacities of 360 mAh/g graphite and 150 mAh/g NMC , the weight ratio of graphite to active material is 0.31/1 g graphite /g AM .…”

“…If one considers a trace water contamination of ≈20 ppm H 2 O in the electrolyte (120 μl of 1 M LiTFSI in EC/EMC, ρ = 1.2 g/cm 3 ) and otherwise fully dried cell components, the total amount of water in the OEMS cell would be 0.16 μmol H2O . Considering a one-electron reduction of water, as shown in Equation 1, the conversion of 1 mol of H 2 O would release 1 mol of OH − and 0.5 mol H 2 .…”

“…17 One of the main differences between the use of graphite and LTO anodes, is the fact, that LTO-containing Li-ion pouch bag cells frequently tend to swell after several cycles or during storage in the charged state, whereby the main gas component is H 2 (≈ 50 wt%). 3,[16][17][18][19][20][21] In contrast to that, pouch cells containing graphite electrodes do not show such strong gassing upon cycling or storage. 13,22 Joho et al 8 performed DEMS measurements on graphite half-cells with different concentrations of water (250 ppm, 1000 ppm and 4000 ppm H 2 O), demonstrating that the amount of C 2 H 4 decreases with increasing H 2 O concentration and that the formation of H 2 increases with the water content.…”

A Liquid Inorganic Electrolyte Showing an Unusually High Lithium Ion Transference Number: A Concentrated Solution of LiAlCl4 in Sulfur Dioxide

“…Furthermore, sufficient solubility of the salt and a high conductivity of the resulting electrolyte solution are required [4,5]. Other major requests include fast formation of a protecting solid electrolyte interphase (SEI) at the anode [7][8][9] and protection of the aluminum current collector from anodic dissolution [10,11]. Additionally, low costs of the electrolyte and non-toxicity matters for high volume production and consumer application.…”