Chromium Oxide On Lithium Manganese Oxide Material

Published Date: 02 Nov 2017

Objective/Goal:

Island growth of Chromium oxide on Lithium manganese oxide material via Atomic layer deposition to enhance Charge-Discharge capacities.

WHY?

LiMn2O4 is of great interest for the replacement of LiCoO2 in Li-ion batteries due to its high voltage, natural abundance, low cost, and environmental benignity [1].

Issues associated and impeding its commercial use: capacity fading, Mn dissolution at high temperature. The capacity fading is primarily due to the following three factors [2],[3] :

Dissolution of Mn3+: Upon discharge, the concentration of Mn 3+ is at a high level. The Mn 3+ may be disparate at surface accordance to equation [4] :

2Mn3+Solid→ Mn4+solid + Mn2+solution

Mn2+ from this dissolves in the electrolyte solution.

Jahn–Teller effect : Jahn – Teller theorem describes the geometrical distortion of ions and molecules that are related to electronic configurations. It occurs after discharge usually on the surface then could spread into the overall composition. Leading to formation of a tetrahedral structure which is low in symmetry and high disorder.

De-lithiated particles are highly unstable .

Amphoteric coatings on LMNO [1] has been carried with Al2O3, ZnO, ZrO2,MgO [3] to improve electrochemical performance.

Chromium oxide coating on LMNO [1] , a stoichiometric amount of Cr3(NO3).9H2O is dissolved in distilled water to which the bare LMNO particles were added.

Cyclic performance test was carried out at room temperature and elevated temperature ( 55oC) .

(a) (b)

Fig 1:(a) Cycling performance and (i) the bare LiMn2O4, (ii) 0.5 wt.% Cr2O3- coated LiMn2O4, (iii) 1

wt.% Cr2O3-coated LiMn2O4, (iv) 2 wt. % Cr2O3-coated LiMn2O4, (v) 3 wt. % Cr2O3-

coated LiMn2O4, The applied current density is 148 mA g−1 (1 C-rate) at room temperature.

Li metal was used as the anode

(b) Cycling performances of (i) the bare LiMn2O4 and (ii) the 1 wt. % Cr2O3-coated LiMn2O4 at a

current level of 1 C (148 mAg −1) in the voltage range of 3.5–4.5 V at elevated temperature

(55 °C).

Table 1: Discharge capacity performance of the base LiMn2O4 and surface treated LiMn2O4cells

Cathode materials

Discharge capacity (mAh g-1)

Capacity fading

1

10th

20th

30th

40th

50th

60th

70th

LiMn2O4

118.1

110.8

107.5

104.6

103

99.3

99.5

99.7

15.5%

0.5%Cr2O3 coated

109.5

106.7

106.7

104.3

103.8

102.5

100.6

100.4

8.3%

1% Cr2O3 coated

107.5

106.7

104.9

102.9

102.1

101.7

101.7

101.7

5.4%

2% Cr2O3 coated

102.7

98.9

98.2

96.9

96.1

95.3

94.5

94.9

7.5%

3% Cr2O3 coated

98.2

95.8

95.8

95.4

95.4

94.5

93.7

92.9

5.4%

Loss of discharge capacity at the last cycle is compared with that at minimum discharge capacity

Chromium oxide coated particles exhibited lower capacity fading compared bare electrode.

Limitation of Chromium:

Chromium is expensive[5] as well as toxic [5, 6]..

Methodology for Atomic layer deposition of Cr2O3 on LMNO particles :

Choice precursor are [7]

PRECURSOR A

PRECURSOR B

REMARK

Chromium(III) Acetylacetonate

Oxygen

Chromium(III) Acetylacetonate would be in solid state at room temperature and needs to heated upto 210 o C ( Melting point ).

Substrate temperature : The temperature at which ALD would be performed on the LMNO particles is 200o C [8] , this would be achieved and maintained via Infrared Lamp placed parallel to the reactor column.

Fluidization: Is done via gas fluidization where Nitrogen gas ( fluidization gas and precursor carrier gas ) is introduced form the bottom of the reactor column .

Minimum fluidization velocity : Minimum velocity of the carrier gas necessary for fluidization of particles is theoretically calculated and attuned until proper fluidization is achieved.

It is calculated via the Ergun equation [9] :

Minimum fluidization velocity equation is modified [10] :

)1/2 , m/sec = 0.25 m/sec

Where,

Symbol

Name

Value

Remarks

ɸs

Spherecity of particles

1

Dp

Diameter of Particles

5 µm

Provided 2-8 µm

g

Gravity

9.8 m/sec2

ρp

Density of particles, LMNO

4100000 g/m3

At 25o C

ρf

Density of fluids, N2

1165 g/m3

At 25o C

ϵmf

Voidage

0.88[11]

Reactor Column:

The particles taken in the reactor would be the sum of particles required for synthesis of the cathode and the particles that would be subjected to characterization.

The reactants would be carried into the reactor column via the nitrogen gas ( carrier gas) in a sequential method i.e , Precursor A : Cr(acac)3 then Precursor B : O2 followed by purge of N2 to carry out the residual gas to the exhaust.

Exposure time

The time set for the exposure between LMNO particles and reactants is Precursor A : 2 minutes and for Precursor B : 2 minutes , followed by a 10 minutes purge time.

Batter assembly:

The cathode material is synthesized and assembled along with the rest of the battery components at the NSM facility.

Battery Testing:

Li-ion battery would be tested for several cycles of charge and discharge at temperatures from – 30o C to + 60o C.

Recommended Methodology

As it is uncertain if the deposited material is Chromium (III) oxide the following technique is proposed.

Carry out ALD at cycles 5, 10. Perform ICP-OES on the processed particles to identify the deposited material and intensity would indicate concentration, if chromium oxide is identified then XPS characterization would indicate oxidation state (Chromium (III) state is desired).

We can go up to 10 % of Chromium oxide coating [2] .

Coating of 2 %, 5%, 10 % Cr2O3 [3] is proposed. Each coated cathode would be subjected to several cycles of charge - discharge, subsequent capacity comparison would be carried out among various cathode coated batteries performance ( a bare electrode battery could also be tested for the same number of cycles which would signify the implications of chromium oxide coating/island deposition ).

Material Characterization technique’s opted and their uses:

Various material Characterization techniques are opted to understand the surface morphology

Particle Characterization:

TEM: Understand the surface features, shape, size and structure.

SEM: Coating evaluations.

EDX: Understand the true composition of the sample (is coupled with TEM).

XPS: Peaks would help us identify the oxidation state of chromium oxide deposited for us Chromium (III).

ICP-OES: Determine the concentration.

Battery Characterization:

XRD: Characterization of the crystalline state.

AFM: Surface Roughness