Blended Electrodes for High Performance Lithium-Ion Batteries
5.2. Background and Motivation
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Chapter 5
LMR-NMC-Carbon Coated-LiMnPO 4
Blended Electrodes for High
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2.5 and 4.8 V [1-5]. Besides, this material is thermally safer than Li[Ni1/3Co1/3Mn1/3]O2 (NMC) cathode material due to their lower cobalt content [1, 3]. The capacity of LMR-NMC closely matches with that of graphite anode [3]. But LMR-NMC has numerous disadvantages such as high irreversible capacity, low capacity retentions and energy loss in this material during cycling which limits its application in developing high energy density LIBs to power EVs [1-12]. The energy density reduces from 1000 Wh kg-1 to 700 Wh kg-1 during 1-200 cycles [4-5]. The energy loss occurred in LMR-NMC is due to the voltage decay because of transformation of layered to the spinel structure of LMR-NMC [4-10]. As discussed in Chapter 3 and 4, transition metal layer migration to the lithium layer leads to transformation of layered to spinel structure [5-9]. The structural transition causes voltage decay from 3.7 V to <3 V operating voltage during discharge which causes significant loss of energy density [4-5]. Moreover, during high voltage cycling, salt decomposition products from the electrolyte deposits on the surface of the cathode, increases the cell impedance and leads to capacity fade [12]. Besides, gradual capacity fade is due to the dissolution of transition metal ions such as Mn [13-14]. So there is intense need to stabilize the material and its interface to mitigate capacity and energy loss.
Many strategies have been followed for the mitigation of the voltage decay and improving capacity retention in LMR-NMC. In general, surface modifications (Chapter 3 and 4) [5, 15-34], cations doping such as chromium, nickel, ruthenium etc.
[16-19] and anions such as fluorine [20-21] or both cation and anion such as Mg and F have been doped in LMR-NMC to improve the electrochemical performance [22].
As discussed in Chapter 3, LiF coating/ F-doping onto LMR-NMC cathodes deliver high capacity of ~300 mAh g-1 at C/10 rate (10-20% greater than the pristine LMR- NMC cathodes), have high discharge voltage plateau (> 0.25 V ) and low charge voltage plateau (0.2 to 0.4 V) compared to pristine LMR-NMC cathodes [21]. Beside, irreversible capacity, voltage fade, capacity loss are significantly reduced in-relation to the pristine LMR-NMC electrodes. F-doped LMR-NMC partially replaces M-O bonds of the material by M-F bonds thereby increasing interfacial stability and high voltage stability over 200 cycles [21]. Besides, both Mg and F (Chapter 4) doping helps to mitigate capacity loss, reduces irreversibly capacity from >25 % to < 10 %,
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increases C rate performance and cyclability compared to pristine LMR-NMC. Both Mg and F doping reduces charging voltage, i.e., increases surface stability and conductivity thereby delivers high capacity at low voltages. Both Mg and F doping reduces structural transition from layered to the spinel phase thereby reducing voltage drop and loss of energy during high voltage cycling [22].
In this Chapter, we made another attempt to overcome the issues of capacity loss, irreversible capacity and voltage decay of LMR-NMC through blending.
Blending is one of the most classical and practical strategy to overcome the drawbacks associated with both the blended cathode materials and get most optimized electrochemical performance than that is possible with individual cathode materials [23-34]. Oxide cathode materials such as xLi2MnO3·(1−x)LiMO2 [26], LiNi1/3Mn1/3Co1/3O2 (NMC) [27], LiNi0.8Co0.15Al0.5O2 (NCA) [28], LiMn2O4 (NMS) [29-31] etc. are blended each other or with olivine type LiFePO4 [23-25] active materials to meet different design requirements such as improved interfacial stability, reduced irreversible capacity, increased cycle life, C-rate performance (pulse power operation), and safety as olivine compounds contain much stronger P-O bonds than other oxide materials [35-37]. In 2001, Numata et al. suggested an approach to improve the capacity retention, reduce Li loss, Mn dissolution of NMS based electrodes by simply mixing NMS and LiNi0.8Co0.2O2 together [29]. In a similar report, blending of NMC with NMS helps to suppress Mn dissolution from NMS, which is a major factor of capacity loss and coulombic inefficiency of the NMS/Li cells [30]. Blending the olivine LiFe0.3Mn0.7PO4 (LFMP) with spinel LiMn1.9Al0.1O4
(LMO) shows high capacity of LFMP with the rate capability of the spinel [32]. In a recent report by Manthiram and coworkers shows blending of LMR-NMC with spinel Li4Mn5O12 or LiV3O8 eliminates the irreversible capacity loss completely at 30 wt. % Li4Mn5O12 and 18 wt. % LiV3O8. The elimination is due to the ability of Li4Mn5O12
and LiV3O8 to insert the extracted lithium that could not be inserted back into LMR- NMC [33]. Blending of 20 wt. % LiFePO4 with LMR-NMC have achieved high energy density and pulse power capability [26].
In this Chapter, we present blending of LMR-NMC with carbon coated LiMnPO4 to overcome the issues of irreversible capacity loss, low capacity retention
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and improved interface instability of LMR-NMC thereby reducing voltage decay and dissolution. LiMnPO4 is an attractive high voltage cathode with an operating voltage of 4.1 V vs. Li/Li+, offers more safety features compared to transition metal oxide cathode due to the strong P–O bond [35-37]. But it has low practical capacity (155 mAh g-1 atC/10 rate) and low ionic and electronic conductivity which could limit the battery performance. Poor ionic and electronic conductivity of LiMnPO4 can be mitigatedthrough synthesis of nanoparticles with in-situ/ex-situ carbon coatings. 20%
of carbon coated LiMnPO4 blending onto LMR-NMC could further improve rate performance; cycle life, thermal and interfacial stability. The structural, morphological, electrochemical performance of blended composite electrode have been investigated in this manuscript. The goal of this work is to demonstrate the probability of blending of two Manganese based positive electrode materials but not to present an optimization study.