I am doing Ni-NTA chromatography and getting little contamination.

• Firstly, I don't think you need to increase the concentration of NaCl so much. Little changes between the 100 to 500 mM range are more than enough. A high concentration of NaCl disrupts the structure of the protein, as it acts as a chaotropic reagent (although not that prevalent).
• Secondly, many optimizations are required if you are trying to get a purified protein just by single-step chromatography (IMAC in your case) unless your protein is going in the pellet post-lysis. In my experience, you should add at least one more purification step to increase the purity of the protein, such as GFC or IEX.
• The only time I experienced the buffer concentration having any effect was when I accidentally increased the Tris concentration up to 50mM and observed that the temperature-dependent changes in the pH of the Tris solution were significantly decreased. 20mM HEPES should be more than enough. You can check the stability of your protein in different buffers, though.
• You can tighten the elution cut (the concentration range of immidazole at which your protein is eluted). For example, if you're doing benchtop, and eluting the protein at 250mM, run a gel for the elution of 200 to 300 mM imidazole. Analyze if, for example, 200 mM elution concentration has more of your protein than the contaminants.
• Adding DTT or 2-Me not only stabilizes the protein but reduces contaminants too.
• If all this fails, design a 3-step purification protocol where IMAC is followed by IEX (after buffer exchange) and GFC. Reach out in case you need any help.

I am using 50mM imidazole in the lysis buffer itself, then I give 4 washing of 50mmM then, 70mM, 90mM, 100mM(twice), 150mM, 200mM, 250mM, 300mM, 350mM, 400mM, 450mM, 500mM. I get the maximum of my protein in the fractions of 100-300mM with contaminations being almost of same concentrations as in all fractions.

Thank you for your kind suggestions, I will try next purification step after IMAC.