Solid State Ionics - Xiamen University

1 Solid State Ionics 283 (2015) 109 114. Contents lists available at ScienceDirect Solid State Ionics journal homepage: Enhanced ionic conductivity of with addition of lithium borate Dawei Wang a, Guiming Zhong a, Yixiao Li a, Zhengliang Gong b, Matthew J. McDonald a, Jin-Xiao Mi c, Riqiang Fu d, Zhicong Shi e, Yong Yang a, . a Collaborative Innovation Center of Chemistry for Energy Materials, State Key Lab of Physical Chemistry of Solid Surface and Department of Chemistry, College of Chemistry and Chemical Engi- neering, Xiamen University , Xiamen 361005, China b College of Energy, Xiamen University , Xiamen 361005, China c Department of Material Science and Engineering, Xiamen University , Xiamen 361005, China d National High Magnetic Field Laboratory, 1800 E.

2 Paul Dirac Drive, Tallahassee, FL 32310, USA. e School of Materials and Energy, Guangdong University of Technology, Guangzhou 510006, China a r t i c l e i n f o a b s t r a c t Article history: A series of lithium borate added electrolyte compounds, xLi3BO3-(1 x) (0 x ), are synthe- Received 21 July 2015 sized and characterized. This so-called LISICON electrolyte system is analyzed by using X-ray diffraction (XRD), Received in revised form 22 October 2015 scanning electron microscopy (SEM), Solid State nuclear magnetic resonance (ss-NMR), electrochemical imped- Accepted 26 October 2015. ance spectra (EIS), and direct current (DC) polarization methods.

3 From 11B MAS NMR spectra, it is demonstrated Available online 7 November 2015. that a small fraction of boron exists in the form of BO4, while its majority settles at grain boundaries in the form of Keywords: BO3, indicating that lithium borate glasses play a role as sintering assistant. This prominently increases the rela- Solid electrolyte tive density of samples, and is bene cial to the ionic conductivity. Further results show that the electrical conduc- Lithium ion battery tion of lithium borate added LISICONs is dominated by Li+ ions, with a transference number of t+ Li and a LISICON corresponding ionic conductivity of about 10 6 S cm 1 at room temperature, almost two times that of pris- Sintering assistant tine Lithium borate 2015 Elsevier All rights reserved.

4 1. Introduction with LiPON glasses, which also showed an ionic conductivity on the order of 10 6 S cm 1 at RT [5]. Inorganic Solid electrolytes have attracted much interest as the range In order to increase the ionic conductivity in the Li4 SiO4 Li3PO4 sys- of applications of lithium ion batteries widens. The replacement of dan- tem, Zhang et al. [6] added small amounts of LiBO2 and its precursors to gerously ammable organic electrolytes by non- ammable, inorganic the system and investigated their effects. They Solid electrolytes can go a long way to ameliorating the safety problems found that the ionic conductivity showed a moderate increase (less that have plagued lithium ion batteries as of late, and they are expected than 30%) after the addition of LiBO2 and/or its precursors, but then de- to play an important role in the future development of large-scale, Solid creased after further addition of LiBO2 because of the formation of a State lithium ion battery systems with enhanced safety features.

5 Li2 SiO3 impurity phase. The lithium borate was speculated to play a Li4 SiO4 Li3PO4 Solid solutions have been reported as lithium ion role as a sintering assistant, but no direct evidence was given in their conductors with a -Li3PO4 structure [1 4]. With an ionic conductivity study. Much earlier, Masquelier et al. [7] had successfully doped silicon of 3 10 6 S cm 1 at room temperature (RT), the composition with boron up to x = in Li4+xBxSi1 xO4 Solid solutions and con- had the best showing of all the Solid solution compounds rmed the formation of Solid solutions from a linear decrease in crystal xLi4 SiO4 (1 x)Li3PO4. In addition, the Li4 SiO4 Li3PO4 system showed parameters after the addition of boron.

6 However, further measurements excellent chemical and electrochemical stability, which was paired on ionic conductivity were not taken in their study. with relatively simple and convenient synthesis procedures. For these In this work, in order to study the form of boron in the Li4 SiO4 Li3PO4. reasons, Li4 SiO4 Li3PO4 Solid electrolytes are capable of competing phase, or more precisely, whether boron atoms insert into the crystal structure or play a role as a sintering assistant, we perform a systematic study on the formation and microstructure of a series of compounds with different ratios of B:( ), using various characterization tech- niques such as XRD, SEM and Solid - State nuclear magnetic resonance Corresponding author.

7 : +86 592 2185753. (NMR) techniques. We further discuss the origin of the improved ionic E-mail address: (Y. Yang). conductivity of the compounds. 0167-2738/ 2015 Elsevier All rights reserved. 110 D. Wang et al. / Solid State Ionics 283 (2015) 109 114. 2. Experimental solutions with each other in a wide stoichiometric range. The usual formulas are (1 x)Li4 SiO 4 xLi3 PO4 (0 b x ) with a Li4 SiO 4. A series of xLi3BO3 (1 x) (0 x ) compounds structure and (1 x)Li3 PO 4 xLi 4 SiO 4 ( x b 1) with a -Li3PO4. were prepared via a conventional Solid reaction. The starting materials structure [1]. Since boron shows only a slightly smaller ionic radius Li2CO3, H3BO3, NH4H2PO4 (purities of , and respective- ( ) than silicon ( ) and phosphorus ( ) while tetrahedral- ly, all from the Sinopharm Chemical Reagent Co.)

8 , Ltd) and SiO2 (purity of ly coordinated [11], it could possibly replace Si and/or P by B and form a , Alfa Aesar) were mixed together and ball-milled using ethanol as series of Solid solutions. In addition, lithium borates are good sintering a grinding reagent. To compensate for lithium loss during calcination at assistants which can improve the bulk properties of ceramic materials. high temperature, 5 mol% extra lithium was added for lithium borate Thus, it is necessary to identity the role of boron in added and 10% for the pristine sample. The precursors Fig. 1 shows the XRD patterns of lithium borate added samples. The were dried after 4 h of ball-milling, then calcined at 900 C for 10 h.

9 Diffraction results indicate that the main sample phase was crystallized This was followed by grinding, with the obtained powders being in an orthorhombic structure, which is the structure of crystal Li3PO3. pressed into pellets under 5 MPa of pressure. For the lithium borate Traces of Li2CO3 were detected due to the extra lithium source. No crys- added samples, pellets were sintered at a temperature of 850 C for talline lithium borates could be found from the XRD patterns, and the 10 h and were then annealed to RT in the furnace, while the pristine indexed peak positions were negligibly shifted with the addition of lith- pellets were sintered at 1000 C for 10 h and were ium borate.

10 In order to investigate in more detail, Jade 5 software was annealed to RT in accordance with the procedures in the literature [1,2]. used to calculate the lattice parameters of the samples. It was found X-ray powder diffraction (XRD) technique was used to characterize that the lattice parameters were not consistently changed with in- the phases and their purities, using Cu K radiation ( ) on a creased boron content and any variations were due to experimental Panalytical X' Pert (Philip, Netherlands) instrument. Normally, XRD pat- error. terns were collected from 10 50 , with a step of , and 10 s per It is dif cult to conclude whether the boron played a role as a substi- step.