| Improving the performance of hydrogen storage alloys is necessary for achieving higher performance MH/Ni batteries.Rare earth-based hydrogen storage alloys have been widely used as cathode materials because of their superior performances compared to those of other hydrogen storage alloys.However,their poor power performance and relatively high cost limit the use of AB5 type hydrogen storage alloy batteries as power sources.To solve the problems associated with rare earth-based AB5 type hydrogen storage alloys by reducing their cost and improving their dynamic performances,In our work,we carried out the investigation of hydrogen storage alloy on three aspects:Co replaced by Cu,Be replaced by Co and Be-Cu replaced by Co.The phase structures and the long-circle dynamic performances of rapidly solidified MLNi3.55Co0.45Mn0.4Al0.3Cu0.3 alloys were investigated.(1)An alloy solidified at 103 K/s exhibited a single LaNis type phase structure,while an alloy solidified at 10 K/s had a dual-phase structure,which was mostly LaNis type with a minor phase that had a nearly A2B7 structure with an RE composition(Ni Co Mn A1 Cu)of 1:3.19.The alloy electrode solidified at 103 K/s exhibited a maximum capacity of 315.9 mAh/g,which was higher than that of the alloy electrode solidified at 10 K/s(306.1 mAh/g).(2)During a 100 cycle process,the I0 of the alloy solidified at 103 K/s increased from 313.81 mA/g(10 cycles)to 1471.18 mA/g(100 cycles),while the I0 of the alloy solidified at 10 K/s increased from 262.03 mA/g(10 cycles)to 1304.49 mA/g(100 cycles).The Rct of the alloy solidified at 103 K/s decreased from 16.64 m?·g(10 cycles)to 3.5 M?·g(100 cycles),while that of the alloy solidified at 10 K/s decreased from 19.92 m?·g(10 cycles)to 4.0 m?·g(100 cycles).The D of the alloy solidified at 103 K/s increased from 3.14 × 10'6 cm2/s(10 cycles)to 2.47 × 10-5 cm2/s(100 cycles),while that of the alloy solidified at 10 K/s increases from 1.3 × 10-6 cm2/s(10 cycles)to 2.04 × 10-5 cm2/s(100 cycles).The influence of beryllium(Be)content on the alloys' phase structures,the batteries'performances,and the dynamic performances of MLNi3.55Co0.75-xMn0.4Al0.3Bex(x =0.00,0.15,0.30,0.45,0.60,and 0.75)alloys were investigated.(1)After Be was added to the alloy,multiple phase structures appeared,the Re:(Ni Co Mn Al)in the matrix phase is 1:5,the Re:(Ni Co Mn Al)in the gray phase is 2:7,and La-enriched and Ni-enriched segregative phases were observed at phase boundaries.As the Be content was increased,the number of segregative phases and the corresponding area gradually increased.(2)With the content of(Be)increusd the activation performance.When the x=0.45,the alloy achieved maximum capacities in it first cycle and the maximum value is 283.9 mAh/g.The alloys' decay rates,which gradually dropped from 1.25 mAh/g-cycle(x = 0.00)to 0.9 mAh/g-cycle(x =0.45),decreased with increasing Be content before increasing with increasing Be content at larger values of x(1.21 mAh/g-cycle at x = 0.75).The alloys' rate capabilities increased with increasing Be content;when discharged at 1625 mA/g,the rate capability increased from 26%(x = 0.00)to 42%(x = 0.60)before decreasing to 39%(x =0.75).(3)The alloys were studied using AC impedance measurements,cyclic voltammetry,the galvanostatic intermittent titration method,the linear polarization curve method,and the Tafel polarization curve method to determine their dynamics.Similar results with small variations were obtained from all of these methods,because of the different formula derivations and principles.When the Be content increased,the exchange current,I0,obtained from AC impedance measurements increased from 131.97 mA/g(x = 0.00)to 519.81 mA/g(x = 0.75).Meanwhile,the charge transfer resistance,Rct,decreased from 39.56 m ?·g(x = 0.00)to 10.04 m ?·g(x = 0.75),and the hydrogen diffusion coefficient,D,increased from 3.08 × 10-11 cm2/s(x = 0.00)to 8.85 × 10-11 cm2/s(x = 0.75).The influence of the beryllium-copper(Be-Cu)content on the phase structure,battery performance,and dynamic performance of the MLNi3.55Co0.75-xMn0.4Al0.3Bex(x = 0.00,0.15,0.30,0.45,0.60,0.75)alloys were also investigated.(1)After Be-Cu was added to the alloys,a variety of phase structures were observed,and La-enriched and Ni-enriched segregative phases appeared at the phase boundaries.When the Be-Cu content increased,the number of segregative phases and corresponding area also increased.The unit cell volume underwent clear changes,increasing from 89.43 A3(x = 0.00)to 89.72 A3(x = 0.45)before decreasing to 89.38 A3(x = 0.75).(2)The hydrogen storage alloy without Be-Cu achieved a maximum capacity in its second activation cycle of 308.3 mAh/g,while those with Be-Cu achieved maximum capacities in their first activation cycles.As their Be-Cu content was gradually increased,the alloys achieved maximum capacities as high as 321.9 mAh/g(x = 0.45)before decreasing to 313.7 mAh/g(x = 0.75).The decay rate of the alloys gradually decreased frorm 1.29 mAh/g-cycle to 1.06 mAh/g·cycle before increasing to 1.27 mAh/g·cycle at high Be-Cu content.The high rate discharge(HRD)capability of the alloys increased as the Be-Cu content increased.When discharged at 1625 mA/g,the HRD1625 increased from 64.95%(x =0.00)to 79.08%(x = 0.60)before decreasing to 78.15%(x = 0.75).(3)According to AC impedance measurements,I0 increased from 98.66 mA/g(x = 0.00)to 214.83 mA/g(x = 0.45)before decreasing to 112.68 mA/g(x = 0.75).Meanwhile,Rct decreased from 52.92 m?·g(x = 0.00)to 24.3 m?·g(x=0.45)and then gradually increasedd to 46.34 m?·g(x = 0.75)with an increase in Be-Cu content.D increased from 1.46 × 10-11 cm2/s(x = 0.00)to 7.41 × 10-11 cm2/s(x=0.45)and then gradually decreased to 4.51 × 10-11 cm2/s(x=0.75)with an increase in the Be-Cu content.The influence of the annealing treatment on the microstructure and electrochemical properties of MlNi3.55 Co0.3Mn0.4Al0.3(Cu0.96Be0.04)0.45 alloy was studied.The Alloys were annealed at 923,1023 and 1123 respectively,and the treatment time was(4h).The results shown that when the annealing treatment temperature increased from 923(4h)to 1023K(4h)to 1123K(4h),the La phase of alloys decreased,the Ni phase of alloys incrsed,the discharge capacity decreased form 321.9 mA/g to 305.43mA/g or 314.58mA/g,the capacity decay rate decreused form 1.03mAh/g.cycle(923K)to 0.95mAh/g.cycle(1023K)or 0.98mAh/g.cycle(1123K),Hower,the HRD1625 of the alloys had no obvious changes.At last,the influence of the annealing treatment time on the characed was 1023K and the treatment time incrensed form 2h,4h and 6h.When the annealing trentment time increased form 2h to 4h or 6h,the white la phase of alloys decreased,the Ni phase and gray A2B7 increased,the discharge capacity decreasd form 321.9mA/g(none)to 308.4mA/g(2h),314.58mA/g(4h)and 310.54mA/g(6h),and HRD1625 had no obvious changes. |
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