Effect of Mn and Cu Addition on Lithiation and SEI Formation on Model Anode Electrodes

Abstract

The effect of addition of Mn 2+ to the electrolyte on the lithiation of a model battery anode was studied using voltammetry, electrochemical quartz crystal microbalance (EQCM), scanning electron microscopy (SEM), Auger electron spectroscopy (AES) and matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS). Cyclic voltammetry of a Au anode showed that the presence of one equivalent monolayer of Mn 2+ in the electrolyte reduces the battery capacity by as much as 48%, a result which is recapitulated in SEM images of the anode surface after five cycles which show that the presence of Mn 2+ blocks lithiation of the anode. AES demonstrates the presence or lack of Mn on the surface. EQCM analysis demonstrates greater initial mass gain on the anode with increasing concentration of Mn 2+ in the electrolyte while MALDI-TOF MS shows no observable differences in the solid electrolyte interface (SEI) between the electrolyte with or without Mn 2+ . The addition of Cu 2+ to the electrolyte exhibits an effect similar to Mn 2+ addition.Lithium ion batteries are a technology of great interest for applications in portable electronics and electric cars due to their high operating voltage and high energy density. LiMn 2 O 4 is a desirable cathode material for a lithium ion battery due to the low cost, environmentally benignity, thermal stability, and abundance. 1-4 However, batteries using LiMn 2 O 4 cathodes are subject to severe capacity fade with cycling. 3,4 This capacity fading is attributed to three main factors:(1) the instability of the electrolyte at high potentials when charging cells, (2) a Jahn-Teller distortion of the MnO 4 center at the discharged state, converting from a cubic symmetry to a tetragonal symmetry, reducing the stability of the cathode structure with cycling, and (3) the dissolution of Mn into the electrolyte. [5][6][7] The dissolution of Mn has been considered a vital issue to be solved to improve LiMn 2 O 4 cathodes. The amount of Mn dissolved into the electrolyte from a LiMn 2 O 4 cathode has been quantitatively analyzed by ion chromatography and concentrations up to 31 ppm Mn 2+ are found. 8 The dissolution of Mn may be enhanced with an increase of the concentration of acid, particularly HF forming from hydrolysis of LiPF 6 , in the electrolyte. 9 Prior research has examined the effect of Mn 2+ in the electrolyte. Mn dissolution reduces battery capacity by two methods: (1) material loss at the cathode 10,11 and (2) Mn reducing at the anode, causing an increase in cell resistance. 12-14 Mn reduction on the anode has been studied on graphite using EDS, XRD, AFM, XPS, and electrochemical impedance spectroscopy. 10,12,14,15 Additionally, a variety of methods have been used to reduce the effect of the dissolved Mn ions. These methods include adding inorganic electrolyte additives to prevent further reduction of Mn on the anode, 16,17 substituting a fraction of the Mn atoms with other transition metal atoms in the cathode, 18 and redepositing the Mn on the cathode before d...

Controlling Interfacial Properties of Lithium‐Ion Battery Cathodes with Alkylphosphonate Self‐Assembled Monolayers

“…This capacity fade is attributed to both chemical corrosion by HF species formed from hydrolysis of LiPF 6 and the release of Mn 2+ species into the electrolyte as the crystal lattice of LMO expands upon discharge. The Mn 2+ species in the electrolyte interact with the SEI on the anode and/or plate on the anode itself, causing performance degradation …”

“…Although the natural SEI affords kinetic stability to the lithium‐ion battery system, other degradative pathways are still present, particularly at the cathode. For example, in lithium manganese oxide (LMO) cathodes, capacity fade is related to the release of Mn 2+ into the electrolyte . Several different modifications of the LMO surface have been developed to prevent this dissolution, including surface oxides, thin gold shells, and graphene sheets .…”

Electrolytes and Interphases in Li-Ion Batteries and Beyond

“…Both groups proposed that the reduced Mn or Cu deposited on anode surface in the form of a monolayer metallic coating that is impermeable to Li + . 189,190 However, this conventional wisdom was recently challenged by Zhan et al, 186 who convincingly proved that Mn(II) species on the anode surface remained at +2 oxidation state without being reduced into metallic form Mn(0). What most likely occurred should be a metathesis process, in which Li + -embedded in SEI was displaced by Mn 2+ , resulting in an ion-insulating interphase populated by Mn 2+ that constitutes an additional barrier to Li + -transport.…”

Degradation Mechanisms and Mitigation Strategies of Nickel-Rich NMC-Based Lithium-Ion Batteries

“…deposited onto graphite anodes, both of which lower storage capacity [38]. Researchers have also proposed another mechanism of Li + that suggests that even monolayer Mn 2+ can impede the movement of Li + through the graphite and the SEI [111]. As for PF 5 , it is produced from the decomposition of various electrolytes and can trigger the open-ring polymerization of EC, which further increases SEI decomposition along with the production of HF [112].…”