Effect of electrolyte and additives on performance of LiNi 0.5 Mn 1.5 O 4

Transcription

1 Effect of electrolyte and additives on performance of LiNi 0.5 Mn 1.5 O 4 Brett L. Lucht Department of Chemistry University of Rhode Island

2 Source of Energy Fade of Lithium-ion Batteries Poor calendar life and performance loss upon thermal abuse, accelerated aging, or extended cycling of Lithium-ion batteries Three primary sources have been reported Thermal decomposition of the electrolyte (LiPF 6 /Carbonate) Thermal deterioration of protective SEI on anode and reactions of electrolyte with anode Reaction of electrolyte with cathode to form surface films composed of LiF/Li x PF y /Li x POF y, PEO, Polycarbonate, ROCO 2 OLi Aurbach, D. J. Power Sources , 497 (2003) Zhang, X. et. al. J. Electrochem. Soc. 148, A463 (2001) Abraham, D. P. et. al. J. Power Sources , 511 (2003) 2

3 Efficiency, % Investigation of Electrolytes for HV Spinel Research suggests two primary mechanisms of performance loss due to electrolyte (LiPF 6 in Carbonates) Oxidation of the electrolyte on cathode surface Metal ion dissolution due to acidic electrolyte decomposition products Investigation of novel electrolytes to Improve Performance of LiNi 0.5 Mn 1.5 O 4 cathodes cycled to high Voltage Cathode Film forming additives which generate a passivation layer inhibiting electrolyte oxidation Lewis Basic Additives which inhibit Mn dissolution Capacity, mah g Room Temperature Elevated 55 o C Cycle No. Very little capacity fade at RT followed by dramatic capacity fade at 55 o C for graphite/linimno 4 cells Cathode laminate may be damaged (Oh J. Power Sources 215, 312, 2012)

4 Q, mah I, ma Storage Experiments of Ni 0.5 Mn 1.5 O 3 with Electrolyte Investigation of the reactions of the electrolyte with the cathode surface. Ni 0.5 Mn 1.5 O 3 /Li cells were stored at various voltages between 4.0 and 5.3 V vs Li for one week. At low voltages, below 4.7 V, there is very little current flow through the cells. As the voltage surpasses 4.7 V vs Li there is a significant increase in charge flow through the cells. The increased charge flow is consistent with electrolyte oxidation on the surface of the cathode at high potential. Similar results were observed for LiNi 0.5 Mn 1.5 O Q, 4.0 V Q, 4.3 V Q, 4.7 V Q, 5.0 V Q, 5.3 V I, 4.0 V I. 4.3 V I, 4.7 V I, 5.0 V I, 5.3 V time, hr Oxidation of the electrolyte solution (1 M LiPF 6 in 1:1:1 (EC:DEC:DMC) at various potentials with a Ni 0.5 Mn 1.5 O 3 /Li half cell Yang, Ravdel & Lucht Electrochem. Solid State Lett 13, A95 (2010) 4

5 Ex-Situ Surface Analysis of LiNi 0.5 Mn 1.5 O 4 cathodes with LiPF 6 in EC/DMC/DEC (1/1/1 vol) 4.0 V O-C C1s C-O O1s Metal Oxide F1s P2p EPDM EPDM 4.0V EPDM 4.3V EPDM 5.0V EPDM 5.3V EPDM, EC free electrolyte Li x PF y O z Li x PF y 4.3 V C=O Li x PF y Li x PF y O z Li x PF y O z PEC 4.7 V O=C 5.0 V 5.3 V Wavenumber (cm -1 ) X Axis Title XPS Suggests Metal oxide covered with polyethylene carbonate (PEC) at high voltage. Polyethylene carbonate and oxalate generation supported by IR 5

6 Impedance before and after cycling at elevated temperature, ET cycling RT cycling ET cycling ET cycling RT cycling (a) (b) RT cycling (c) full cell anode half cell cathode half cell Half cells were prepared from electrode extracted from cells cycled at ET Most of the impedance growth occurs on the cathode 6

7 XPS Spectra of Cycled Cathodes fresh C-O C=O C 1s O 1s C-C Li 2 CO 3 O=C O-M PVDF F 1s P 2p Li x PO y F z Mn 2p Ni 2p Li 2 CO 3 O-C LiF STD RT STD ET Significant electrolyte decomposition occurs upon cycling at 55 o C Decomposition products include polyethylene carbonate and lithium alkyl carbonates and lithium oxalate Mn and Ni content at surface is decreased 7

8 SEM Images of Cathodes Fresh cathode Cathode half cell after cycled at RT The changes to the morphology of the bulk cathodes upon cycling is small Cathode half cell after continuously cycled at ET, and RT again. 8

9 XPS Spectra of Anodes C 1s C-O C-C O 1s PVDF F 1s Li x PO y F z P 2p Mn 3p fresh C=O Li 2 CO 3 Li 2 CO 3 O-C O=C LiF Li x PO y F z STD RT STD ET Significant electrolyte decomposition occurs upon cycling at 55 o C Decomposition products include lithium alkyl carbonates, Li 2 CO 3, LiF, and Li x PF y O z No change to bulk material by SEM, but delamination is observed Mn is also present on the anode surface (ICP-MS) 9

10 Efficiency, % Effect of Electrolyte Additives on Cycling Behavior RT ET RT Capacity, mah g STD Add 1.5% LiBOB Cycle No. 0 Incorporation of electrolyte additive(s) results in dramatic decrease in capacity fade upon cycling at 55 o C for graphite/lini 0.5 Mn 1.5 O 4 cells 10

11 XPS Spectra of Anodes Changes to cathode surface are small upon incorporation of additive 11

12 XPS Spectra of Cycled Cathodes Additive inhibits LiF formation on Anode Mn deposition is also inhibited 12

13 Investigating the performance of LiNi 0.5 Mn 1.5 O 4 cathodes cycled to high voltage Discovered that the two leading sources of performance fade are electrolyte oxidation and Mn dissolution Electrolyte oxidation below 4.9 V lesser contributor, thermal effects are larger contributor. Both the anode and cathode are damaged from cycling at 55 o C. Additives have been developed that inhibit Mn dissolution and improve performance of high voltage cathodes Novel electrolyte formulations can improve the cycling performance of LiNi 0.5 Mn 1.5 O 4 cathodes at high voltage (4.9 V vs Li) 13

14 Acknowledgments BASF SE, Arnd Garsuch Assistant Secretary for Energy Efficiency and Renewable Energy, Office of Vehicle Technologies of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231, Subcontract No under the Batteries for Advanced Transportation Technologies (BATT) Program Boris Ravdel and Frank Puglia, Yardney Technical Products/Lithion. Mike Platek, Sensors and Surface Technology Partnership at URI for assistance with XPS and SEM. 14

15 15