Characterisation of batteries by electrochemical impedance spectroscopy

In the pursuit of batteries with higher energy density and lower cost, central to advancement of the technology is the ability to prolong cycle life. Techniques are sought which can elucidate information on battery degradation without significantly disrupting the performance of cells. Electrochemical impedance spectroscopy (EIS) offers a non-destructive route to in-situ analysis of the dynamic processes occurring inside a battery. The technique is relatively easy to use, but meaningful data analysis requires assignment of spectroscopic features to battery impedance components. Three-electrode cell configurations afford a way to potentially disentangle the impedance components. This paper examines a number of three-electrode cell designs reported in the literature, and compares their advantages and limitations. EIS results obtained using a novel in-house, three-electrode pouch cell are reported and the results compared with those obtained from conventional two-terminal impedance complex plane plots. In this way, the separate contributions of anodic and cathodic impedances can be assessed

Surface treatments of Li1+xMn2-xO4 spinels for improved elevated temperature performance

In this paper, we introduce the use of surface treatments to improve the elevated temperature storage characteristics of the Li1+xMn2-xO4 spinel in Li-ion batteries. Two approaches are introduced, the first consists of the application of an inorganic lithium borate glass composition to the surface, the second utilizes an acetylacetone complexing agent. All surface treatments were found to improve the elevated temperature performance of the Li1+xMn2-xO4 spinel to some degree. Results are discussed with respect to the active failure mechanisms

Origin of self-discharge mechanism in LiMn2O4-based Li-ion cells: A chemical and electrochemical approach

LiMn2O4-based Li-ion cells suffer from a limited cycle-life and a poor storage performance at 55°C both in their charged and discharged states. To get some insight on the origin of the poor 55°C storage performance, the voltage distribution through plastic Li-ion cells during electrochemical testing was monitored by means of 3-electrode type measurements, From these measurements, coupled with chemical analysis, X-ray diffraction and microscopy studies, one unambiguously concludes that the poor performance of LiMn2O4/C-cells at 55°C in their discharged state is due to enhanced Mn dissolution that increases with increasing both the temperature and the electrolyte HF content. These results were confirmed by a chemical approach which consists in placing a fresh LiMn2O4 electrode into a 55°C electrolyte solution. A mechanism, based on an ion-exchange reaction leading to the Mn dissolution is proposed to account for the poor storage performance of LiMn2O 4/C Li-ion cells in their discharged state. In order to minimize the Mn dissolution, two surface treatments were performed. The first one consists in applying an inorganic borate glass composition to the LiMn2O 4 surface, the second one in using an acetylacetone complexing agent

An update on the high temperature ageing mechanism in LiMn2O4-based Li-ion cells

LiMn2O4-based Li-ion cells suffer from a limited cycle-life and a poor storage performance at 55°C, both in their charged and discharged states. From 3-electrode type electrochemical measurements, the non-stability of LiMn2O4 in electrolytes containing traces of HF was identified as being the source of such a poor performance. To get some insight in the mechanism by which the high-temperature ageing proceeds, a survey of the chemical stability of high surface area LiMn2O4 in various Li-based electrolytes was performed as a function of temperature. The growth of a protonated λ-MnO2 phase was identified when LiMn2O4 powders were stored into the electrolyte at 100°C for several hours. Such a protonated phase is partially inactive with respect to lithium intercalation, thereby accounting for some of the irreversible capacity loss experienced at 55°C for LiMn2O4-based Li-ion cells. © 1999 Elsevier Science S.A. All rights reserved

The elevated temperature performance of the LiMn2O4/C system: Failure and solutions

This paper reviews various chemical approaches that have participated in the improvement of the high temperature performance of the LiMn2O4/C Li-ion system. These approaches range from chemical surface and bulk modification of the spinel to the improvement of electrolyte stability towards acidification, and to the stabilization of the SEI chemistry of the carbon anode. More specifically, we describe the advantages of (1) modifying the surface chemistry of the spinel in order to obtain encapsulated particles or (2) modifying the crystal chemistry of the spinel through dual cationic and anionic substitutions by improving its stability towards Mn dissolution. The role of the carbon negative electrode towards the high temperature issue, namely through the formation/dissolution of the SEI layer is discussed, and a way of controlling such an SEI layer through a pre-conditioning of the cell is presented. The benefit of adding zeolites to the Li-ion cell to trap some of the species (H+ , or others) generated during cell functioning as the result of the electrolyte decomposition or SEI layer is presented. Finally, from a compilation of other reports on that topic together with the present work, our present understanding of the failure mechanism in the LiMn2O4 system is elucidated. © 1999 Elsevier Science Ltd. All rights reserved

Self-discharge of LiMn2O4/C Li-ion cells in their discharged state: Understanding by means of three-electrode measurements

The potential distribution through plastic Li-ion cells during electrochemical testing was monitored by means of three- or four-electrode measurements in order to determine the origin of the poor electrochemical performance (namely, premature cell failure, poor storage performance in the discharged state) of LiMn2O4/C Li-ion cells encountered at 55°C. Several approaches to insert reliably one or two reference electrodes that can be either metallic lithium or an insertion compound such as Li4Ti5O12 into plastic Li-ion batteries are reported. Using a reference electrode, information regarding the evolution of (i) the state of charge of each electrode within a Li-ion cell, (ii) their polarization, and (iii) their rate capability can be obtained. From these three-electrode electrochemical measurements, coupled with chemical analyses, X-ray diffraction, and microscopy studies, one unambiguously concludes that the poor 55°C performance is mainly due to the instability of the LiMn2O4 phase toward Mn dissolution in LiPF6-type electrolytes. A mechanism, based on Mn dissolution, is proposed to account for the poor storage performance of LiMn2O4/C Li-ion cells