V. Focused Fundamental Research - EERE - U.S. Department of ...

V.B.10 Development of High Energy Cathode (PNNL)
Zhang, Liu – PNNL
30°C increment up to 534°C in a UHP-Ar atmosphere as lithiated MnPO 4 can be summarized by the following
shown in Fig. 1. We observed MnPO 4 reduction to
reaction:
Mn 2 P 2 O 7 with oxygen evolution at 490°C, which coincides o
o
150~180 C 490 C 1 1
MnPO
4
MnPO
4
Mn
2PO 2 7
O
2
with the phase changes in MnPO 4 H 2 O. Between 180 and Pnma Jahn Teller Distortion 2 C2/ m 2
490°C, the charged MnPO 4 undergoes amorphization
(highlighted yellow area in Figure V - 51). TGA-MS results
Our results demonstrate the intrinsic thermal stability
showed no oxygen released before reaching 490°C.
of electrochemically lithiated or de-lithiated LiMnPO
However, initial CO 2 release between 200~400°C was
4 .
However, the discharge rate of LiMnPO
observed for the charged MnPO 4 electrode consistent with
4 needs to be
improved for their practical application. Because heat
the weight changes observed in the TGA plot indicating
evolution related to the LiMnPO
that the continuous weight loss up to 450°C resulted from
4 cathode is an extrinsic
material property and not an intrinsic property, smart
decomposition of the SEI layer formed on the LiMnPO 4
material design, such as carbon coating, can significantly
electrode surface. CO 2 evolution is commonly observed
reduce surface-electrolyte reactions better than
during oxidation at the SEI layer and catalytic
Li(NiCoX)O
decomposition of carbonate-based organic electrolytes.
2 (X: Mn or Al) compounds in terms of both
onset temperatures and specific heat evolution.
The thermal stability and phase transformation of the de-
Figure V - 51: In situ, hot-stage XRD characterization of (a) the charged MnPO4 electrode and (b) the MnPO4H2O powder under an UHP-Ar atmosphere
(heating rate: 5°C /min).
LiMnPO 4 Synthesized from a Non-Stoichiometric
Li:Mn Ratio. The influences of lithium contents in the
starting materials on the final performance of Li x MnPO 4 (x
hereafter represents the starting Li content in the synthesis
step, which does not necessarily mean that Li x MnPO 4 is a
single phase solid solution in this work) were investigated
systematically. From the results of ICP mass
spectroscopy, the Li:Mn ratio matched very well with the
designed compositions, which confirms that the
precipitation method is a feasible approach for tuning the
lithium content in the final product. Rietveld refinement
of the XRD data shown in Figure V - 52 revealed that
Mn 2 P 2 O 7 is the main impurity when x 1.0. Magnetic and XAS studies
further confirmed that the main phase in Li x MnPO 4
samples was LiMnPO 4 , and the variation in Li content
leads to the formation of additional Li-contained (Li 3 PO 4 )
or Mn-contained (Mn 2 P 2 O 7 ) phases to accommodate the
stoichiometry.
The as-prepared Li 0.8 MnPO 4 , which is a Li deficient
phosphate, is quite different from chemically delithiated
Li x MnPO 4 because other byproducts, such as a Mndeficient
phase, may occur during the interactions between
LiMnPO 4 and the oxidant (usually NO 2 BF 4 dissolved in
acetonitrile), thus influencing or hiding the physical and
electrochemical properties of the material itself.
For Li 0.5 MnPO 4 and Li 0.8 MnPO 4 , gradual increases in
the reversible capacity with cycling were observed, Figure V
- 53, which may be related to interactions between Mn 2 P 2 O 7
and LiMnPO 4 . Among all the samples, Li 1.1 MnPO 4
exhibits the most stable cycling probably because of the
Li 3 PO 4 coating on the surface of LiMnPO 4 nano-particles
that functions as a solid electrolyte to facilitate ion
transport. Therefore the electrochemical performance of
Energy Storage R &D 514 FY 2011 Annual Progress Report